tools/target_dec_fuzzer: Adjust threshold for H264

Fixes: Timeout (too long -> 3sec)
Fixes: 28047/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-4662727980875776

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 46c4f39307)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tools/target_dec_fuzzer.c

@@ -163,6 +163,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_G2M:         maxpixels  /= 1024;  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;
     case AV_CODEC_ID_HAP:         maxpixels  /= 128;   break;
     case AV_CODEC_ID_HEVC:        maxpixels  /= 16384; break;
     case AV_CODEC_ID_HNM4_VIDEO:  maxpixels  /= 128;   break;