tools/target_dec_fuzzer: Update maxpixels_per_frame for low thresholds

Fixes: Timeout
Fixes: 29990/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSS2_fuzzer-5469155073589248

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

@@ -98,7 +98,7 @@ static int audio_video_handler(AVCodecContext *avctx, AVFrame *frame,
 
 // Ensure we don't loop forever
 const uint32_t maxiteration = 8096;
-const uint64_t maxpixels_per_frame = 4096 * 4096;
+uint64_t maxpixels_per_frame = 4096 * 4096;
 uint64_t maxpixels;
 
 uint64_t maxsamples_per_frame = 256*1024*32;
@@ -197,6 +197,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     }
 
     maxsamples_per_frame = FFMIN(maxsamples_per_frame, maxsamples);
+    maxpixels_per_frame  = FFMIN(maxpixels_per_frame , maxpixels);
 
     AVCodecContext* ctx = avcodec_alloc_context3(c);
     AVCodecContext* parser_avctx = avcodec_alloc_context3(NULL);