avcodec/ituh263dec: Fix runtime error: left shift of 1342177279 by 1 places cannot be represented in type 'int'

Fixes: 659/clusterfuzz-testcase-5866673603084288

Huge DMV could be created by an encoder ignoring the spec

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