tools/target_dec_fuzzer: Do not attempt to fuzz VDPAU, its not supported

Fixes: 1364/clusterfuzz-testcase-minimized-6459843441328128
Fixes: 1392

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