avcodec/alsdec: Fixes invalid shifts in read_var_block_data() and INTERLEAVE_OUTPUT()

avcodec/alsdec: Fixes invalid shifts in read_var_block_data() and INTERLEAVE_OUTPUT()

Fixes: left shift of negative value -6
Fixes: 15275/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5742361767837696
Fixes: signed integer overflow: 41582592 * 256 cannot be represented in type 'int'
Fixes: 15296/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5739558227935232
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>