tools/target_dec_fuzzer: Only audio uses the return value to decode packets in pieces, correct the code to match that

Fixes: Timeout
Fixes: 483/fuzz-0-ffmpeg_VIDEO_AV_CODEC_ID_H263I_fuzzer

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
