Fixes: signed integer overflow: 4 * 2307917133220067266 cannot be represented in type 'long'
Fixes: 62164/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-6307690022043648

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