avcodec/vp3: Check remaining bits in unpack_dct_coeffs()

Decreases the time spend decoding junk.

May fix: 1283/clusterfuzz-testcase-minimized-6221126759874560

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