tools/target_dec_fuzzer: Adjust threshold for VP9

The threshold is chosen so that the worse frames would together not take
excessive time.
A better solution is welcome!

Fixes: Timeout (308sec ->102ms)
Fixes: 18314/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VP9_fuzzer-5701689176227840

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

tools/target_dec_fuzzer.c

@@ -163,6 +163,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_TGV:         maxpixels /= 32;    break;
     case AV_CODEC_ID_TRUEMOTION2: maxpixels /= 1024; break;
     case AV_CODEC_ID_VP7:         maxpixels /= 256;  break;
+    case AV_CODEC_ID_VP9:         maxpixels /= 4096; break;
     }