Fixes: signed integer overflow: 539033600 - -1910497124 cannot be represented in type 'int'
Fixes: 30928/clusterfuzz-testcase-minimized-ffmpeg_dem_WTV_fuzzer-5922630966312960
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>