avcodec/mpegaudiodec_template: Fix runtime error: signed integer overflow: 2053224902 + 2053224902 cannot be represented in type 'int'

Fixes: 696/clusterfuzz-testcase-5853632270434304

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