[omx] Fix segfault when reencoding noisy jpegs

See: http://forum.stmlabs.com/showthread.php?tid=15235&pid=107589#pid107589
The fanart has a dotty noise effect that results in very large output jpeg size.
A bug in the code is triggered where we overwrite the encoded jpeg output buffer
Merge pull request #5177 from popcornmix/jpegseg

[omx] Fix segfault when reencoding noisy jpegs