avcodec/srtdec: Fix signed integer overflow: 1811992524 * 384 cannot be represented in type 'int'

Fixes: 617/clusterfuzz-testcase-6413875723370496

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