value temporarily unavailable, due to optimizations in stream

I recently started testing my application using a release build (Manage Schemes and set run from debug to release). What I've noticed is that occasionally I get the following error pop up inside the stream method and I can't seem to figure out a way to find the error. This works perfectly in debug mode, but in the release build I am not getting any notification aside from the SIGABRT message. Nor am I aware of how to check the crash logs on the simulator to see what the issue can be. Below I've attached a stack trace of the crash:

#0  0x918749c6 in __pthread_kill ()
#1  0x916c0f78 in pthread_kill ()
#2  0x916b1ce3 in __abort ()
#3  0x916ae64a in __stack_chk_fail ()
#4  0x00006d0d in -[MainViewController processMessage:len:] (_cmd=0x9ceb9, msg=0xdfd7004 "Login="User" Pass="Pass" Id="1234" PlayerId="345" Location="12,35" Color="Red" PlayerId="65" Location="180,0" Color="Blue" PlayerId="29" Location="0,200" Color="..., len=333295) at /Users/seb/Desktop/Tutorials/Networking/MainViewController.m:850
#5  0x000044b3 in -[MainViewController stream:handleEvent:] (_cmd=0x17a0410, stream=<value temporarily unavailable, due to optimizations>, eventCode=<value temporarily unavailable, due to optimizations>) at /Users/seb/Desktop/Tutorials/Networking/MainViewController.m:260
#6  0x01716501 in _inputStreamCallbackFunc ()
#7  0x016e606d in _signalEventSync ()
#8  0x016e67ca in _cfstream_solo_signalEventSync ()
#9  0x016e5e71 in _CFStreamSignalEvent ()
#10 0x016e6727 in CFReadStreamSignalEvent ()
#11 0x020083ad in SocketStream::dispatchSignalFromSocketCallbackUnlocked ()
#12 0x01f64191 in SocketStream::socketCallback ()
#13 0x01f640a1 in SocketStream::_SocketCallBack_stream ()
#14 0x016b3e44 in __CFSocketPerformV0 ()
#15 0x0171997f in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ ()
#16 0x0167cb73 in __CFRunLoopDoSources0 ()
#17 0x0167c454 in __CFRunLoopRun ()
#18 0x0167bdb4 in CFRunLoopRunSpecific ()
#19 0x0167bccb in CFRunLoopRunInMode ()
#20 0x0226f879 in GSEventRunModal ()
#21 0x0226f93e in GSEventRun ()
#22 0x0066aa9b in UIApplicationMain ()
#23 0x0000201b in main (argc=1, argv=0xbffff5e0) at /Users/seb/Desktop/Tutorials/Networking/main.m:14

Case statement from stream:handleevent in where the crash happens:

case NSStreamEventHasBytesAvailable:
        {
            if (stream == inputStream)
            {
                NSLog(@"NSStreamEventHasBytesAvailable");

                uint8_t buffer[4096];
                unsigned int len = 0;
                NSInputStream* inputstream = (NSInputStream*)stream;
                len = [inputstream read:buffer maxLength:sizeof(buffer)];

                // Check if our stream is still valid
                if([inputstream streamError] != nil)
                {
                    //We lost our connection to the host
                    NSError *theError = [stream streamError];
                    [self setConnectError:[NSString stringWithFormat:@"Error %i: %@",
                                                            [theError code], [theError localizedDescription]]];

                    break;
                }

                [streamIncomingData appendBytes:buffer length:len];

                int processedBytes = 0;
                while (true)
                {
                    if ([streamIncomingData length] >= processedBytes + sizeof(uint32_t))
                    {
                        const char* bufferstart = ((const char*)[streamIncomingData mutableBytes]) + processedBytes;
                        uint32_t sz_n;
                        memcpy(&sz_n, bufferstart, sizeof(sz_n));
                        uint32_t sz_h = htonl(sz_n);

                        if ([streamIncomingData length] >= processedBytes + sz_h + sizeof(uint32_t))
                        {
                            [self processMessage:bufstart + sizeof(uint32_t) len:sz_h];
                            processedBytes += sz_h + sizeof(uint32_t);
                        }
                        else
                        {
                            break;
                        }
                    }
                    else
                    {
                        break;
                    }
                }
                if (processedBytes)
                {
                    if (processedBytes < [streamIncomingData length])
                    {
                        NSMutableData *newdata = [NSMutableData dataWithBytes:[streamIncomingData bytes] + processedBytes length:[streamIncomingData length] - processedBytes];
                        [streamIncomingData setData:newdata];
                    }
                    else
                    {
                        [streamIncomingData setLength:0];
                    }
                }
            }

            break;
        }

processMessage method:

- (void)processMessage:(const char*)msg len:(int)len
{
    NSLog(@"processMessage start (%d)", len);
    {
        char buffer[len + 1];
        memcpy(buffer, msg, len);
        buffer[len + 1] = '\0';
        NSLog(@"%s", buffer);
    }

    NSLog(@"processMessage end");
}

Any advice is greatly appreciated.

You have corrupted the stack by overflowing the buffer by trying to assign the null terminator outside of the bounds len + 1. The correct code would be:

    char buffer[len + 1];
    memcpy(buffer, msg, len);
    buffer[len] = '\0';
    NSLog(@"%s", buffer);

What you were doing was undefined behavior and combining undefined behavior with the optimizations that are done for release mode is most likely the reason you only saw this issue in release mode.