avformat/aiffdec: avoid integer overflow in get_meta()

Fixes: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
Fixes: 45891/clusterfuzz-testcase-minimized-ffmpeg_dem_AIFF_fuzzer-6159183893889024

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


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