tools/target_dec_fuzzer: Adjust motionpixels threshold

Fixes: Timeout (151sec -> 173ms)
Fixes: 16053/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOTIONPIXELS_fuzzer-5647069169057792
Fixes: 16053/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOTIONPIXELS_fuzzer-5108957126852608

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

@@ -140,6 +140,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         // Cliping in C, generally slow even with small input
     case AV_CODEC_ID_INDEO4:    maxpixels /= 128; break;
     case AV_CODEC_ID_LSCR:        maxpixels /= 16; break;
+    case AV_CODEC_ID_MOTIONPIXELS:maxpixels /= 256; break;
     case AV_CODEC_ID_TRUEMOTION2: maxpixels /= 1024; break;
     }