avcodec/mpeg4videodec: Clear mcsel before decoding an image

Fixes: runtime error: signed integer overflow: 2146467840 + 1032192 cannot be represented in type 'int'
Fixes: 2826/clusterfuzz-testcase-minimized-5901511613743104

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