avcodec/eamad: Fix runtime error: signed integer overflow: 49674 * 49858 cannot be represented in type 'int'

Fixes: 1394/clusterfuzz-testcase-minimized-6493376885030912

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