tools/target_dec_fuzzer: Adjust threshold for ZLIB

Fixes: Timeout
Fixes: 49769/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ZLIB_fuzzer-6302938657587200

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

@@ -298,6 +298,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_WMALOSSLESS: maxsamples /= 1024;  break;
     case AV_CODEC_ID_YLC:         maxpixels  /= 1024;  break;
     case AV_CODEC_ID_ZEROCODEC:   maxpixels  /= 128;   break;
+    case AV_CODEC_ID_ZLIB:        maxpixels  /= 4096;  break;
     }
 
     maxsamples_per_frame = FFMIN(maxsamples_per_frame, maxsamples);