avcodec/gsmdec_template: Fix runtime error: signed integer overflow: -22527 * 99113 cannot be represented in type 'int'

Fixes: 636/clusterfuzz-testcase-6520876646268928
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
