avcodec/ituh263dec: Use 0xffff as error code in h263p_decode_umotion()

This matches ff_h263_decode_motion() both functions error codes are interpreted by the same common code

Fixes: 690/clusterfuzz-testcase-4744944981901312

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