avcodec/truemotion2: Fix overflow in tm2_apply_deltas()

Fixes: signed integer overflow: 1077952576 + 1077952576 cannot be represented in type 'int'
Fixes: 7712/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5056281753681920

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>