H264Libav: Use AVFrame for pixel format conversion buffer

H264Libav: Use AVFrame for pixel format conversion buffer

This ensures that the buffer is allocated with the correct alignment and
padding for use with sws_scale.

This fixes out-of-bounds writes which would in some cases cause
segmentation faults and/or heap corruption.

(cherry picked from commit fff17cae7738d6b3d65426a0ce7df76e8034ad1c)