avcodec/mpeg4videodec: Improve the overflow checks in mpeg4_decode_sprite_trajectory()

Also clear the state on errors

Fixes integer overflows in 701/clusterfuzz-testcase-6594719951880192

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