avcodec/vp568: Check that there is enough data for ff_vp56_init_range_decoder()

Fixes: timeout in 730/clusterfuzz-testcase-5265113739165696 (part 1 of 2)

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