Up until now, h264_mp4toannexb would grow the output packet's buffer by
the desired amount every time another NAL unit of the input packet has
been read; this commit changes this: The input buffer is now essentially
parsed twice, once to determine the final size of the output packet and
once to write the output packet's data.

Fixes: Timeout
Fixes: 19322/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_MP4TOANNEXB_fuzzer-5688407821123584
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>