tools/target_dec_fuzzer: Do not test AV_CODEC_FLAG2_FAST with AV_CODEC_ID_H264

This combination skips allocating large padding which can read out of array

Fixes: 20978/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5746381832847360

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: NMichael Niedermayer <michael@niedermayer.cc>

tools/target_dec_fuzzer.c

@@ -206,7 +206,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
             if (flags & 8)
                 ctx->err_recognition |= AV_EF_EXPLODE;
         }
-        if (flags & 0x10)
+        if ((flags & 0x10) && c->id != AV_CODEC_ID_H264)
             ctx->flags2 |= AV_CODEC_FLAG2_FAST;
 
         if (flags & 0x40)