This also makes the code consistent with the existing similar MUL64()
in decode_var_block_data()

Fixes: signed integer overflow: -7277630735906765035 + -3272193951413647896 cannot be represented in type 'long'
Fixes: 16015/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5666552818434048

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