tools/target_dec_fuzzer: Fix return code on open failure

Fixes: 1271/clusterfuzz-testcase-minimized-6095220498235392

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

tools/target_dec_fuzzer.c

@@ -170,7 +170,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 
     int res = avcodec_open2(ctx, c, NULL);
     if (res < 0)
-        return res;
+        return 0; // Failure of avcodec_open2() does not imply that a issue was found
 
     FDBCreate(&buffer);
     int got_frame;