avcodec/xwddec: Check bpp more completely

      Fixes out of array access
Fixes: 1399/clusterfuzz-testcase-minimized-4866094172995584

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