corrects invalid return when avpkt->size == 0.

Reproducible with test case and ffplay -threads 2. Stack trace:
http://pastebin.com/PexZ4Uc0

Test case:
http://commondatastorage.googleapis.com/dalecurtis-shared/crash.ogmSigned-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>