avcodec/ac3dec: Use frame_size if superframe_size is 0

Fixes: Infinite loop
Fixes: 7669/clusterfuzz-testcase-ffmpeg_AV_CODEC_ID_AC3_FIXED_fuzzer-4689042185650176
Fixes: 7670/clusterfuzz-testcase-ffmpeg_AV_CODEC_ID_AC3_fuzzer-4706306762997760
Fixes: 7672/clusterfuzz-testcase-ffmpeg_AV_CODEC_ID_EAC3_fuzzer-4702108499574784

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>