avcodec/dxv: Check for end of input in dxv_decompress_dxt5()

Fixes: Timeout
Fixes: 3291/clusterfuzz-testcase-4630024655208448

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