avcodec/truemotion2: Fix integer overflow in TM2_RECALC_BLOCK()

Fixes: signed integer overflow: 1477974040 - -1877995504 cannot be represented in type 'int'
Fixes: 4861/clusterfuzz-testcase-minimized-4570316383715328

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