buf_size is not updated as buf is advanced so it is wrong after the first
iteration

Fixes: Timeout (160sec -> 27sec)
Fixes: 18658/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_G729_fuzzer-5729784269373440

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 336f9461)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>