tools/target_dec_fuzzer: Adjust threshold for VMNC

Fixes: Timeout
Fixes: 30055/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VMNC_fuzzer-4739482576355328

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

tools/target_dec_fuzzer.c

@@ -187,6 +187,7 @@ 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_VC1IMAGE:    maxpixels  /= 8192;  break;
+    case AV_CODEC_ID_VMNC:        maxpixels  /= 8192;  break;
     case AV_CODEC_ID_VP7:         maxpixels  /= 256;   break;
     case AV_CODEC_ID_VP9:         maxpixels  /= 4096;  break;
     case AV_CODEC_ID_WMV3IMAGE:   maxpixels  /= 8192;  break;