avcodec/dxv: Check remaining bytes in dxv_decompress_raw()

Fixes: Timeout
Fixes: 2006/clusterfuzz-testcase-minimized-5766515037044736

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