tools/target_dec_fuzzer: adjust threshold for gem

Fixes: Timeout
Fixes: 42035/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GEM_fuzzer-5033604191748096

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

@@ -166,6 +166,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_FLAC:        maxsamples /= 1024;  break;
     case AV_CODEC_ID_FLV1:        maxpixels  /= 1024;  break;
     case AV_CODEC_ID_G2M:         maxpixels  /= 1024;  break;
+    case AV_CODEC_ID_GEM:         maxpixels  /= 512;   break;
     case AV_CODEC_ID_GDV:         maxpixels  /= 512;   break;
     case AV_CODEC_ID_GIF:         maxpixels  /= 16;    break;
     case AV_CODEC_ID_H264:        maxpixels  /= 256;   break;