By using ff_avc_write_annexb_extradata instead of the h264_mp4toannexb
BSF, the code for doing the conversion itself is kept much shorter,
there's less state to restore at the end, we don't risk leaving the
AVCodecContext in an inconsistent state if returning early due to
errors, etc.

Also add a missing free if the base64 encoding fails.
Signed-off-by: Martin Storsjö <martin@martin.st>