tools/target_dec_fuzzer: Compute CLUT for DVBSUB only once

Fixes: Timeout
Fixes: 30845/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVBSUB_fuzzer-5744263725973504

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

@@ -157,6 +157,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_COOK:        maxsamples /= 1<<20; break;
     case AV_CODEC_ID_DIRAC:       maxpixels  /= 8192;  break;
     case AV_CODEC_ID_DST:         maxsamples /= 1<<20; break;
+    case AV_CODEC_ID_DVB_SUBTITLE: av_dict_set_int(&opts, "compute_clut", -2, 0); break;
     case AV_CODEC_ID_DXV:         maxpixels  /= 32;    break;
     case AV_CODEC_ID_FFWAVESYNTH: maxsamples /= 16384; break;
     case AV_CODEC_ID_FLAC:        maxsamples /= 1024;  break;