avcodec/mpegpicture: Keep ff_mpeg_framesize_alloc() failure state consistent

Fixes: null pointer dereference
Fixes: ff_put_pixels16_sse2.mp4

Found-by: Rafael Dutra <rafael.dutra@cispa.de>
Regression-since: 4b2863ff
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>