tools/target_dec_fuzzer: Reduce maxpixels for HEVC

high resolutions with only small blocks appear to be rather
slow with the fuzzer + sanitizers.
A solution which makes this run faster is welcome.

Fixes: Timeout (did not wait -> 17sec)
Fixes: 21006/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-6002552539971584

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

@@ -148,6 +148,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_G2M:         maxpixels  /= 64;    break;
     case AV_CODEC_ID_GDV:         maxpixels  /= 512;   break;
     case AV_CODEC_ID_GIF:         maxpixels  /= 16;    break;
+    case AV_CODEC_ID_HEVC:        maxpixels  /= 16384; break;
     case AV_CODEC_ID_HNM4_VIDEO:  maxpixels  /= 128;   break;
     case AV_CODEC_ID_IFF_ILBM:    maxpixels  /= 128;   break;
     case AV_CODEC_ID_INDEO4:      maxpixels  /= 128;   break;