Enable fuzzing of writing foreign metadata back on decoding

The output file was deleted before writing back foreign metadata
could take place, making that code always fail and not covered by
fuzzing

src/flac/decode.c

@@ -304,10 +304,7 @@ void DecoderSession_destroy(DecoderSession *d, FLAC__bool error_occurred)
 #endif
 		fclose(d->fout);
 
-#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
-	/* Always delete output file when fuzzing */
 		if(error_occurred)
-#endif
 			flac_unlink(d->outfilename);
 	}
 }
@@ -582,6 +579,11 @@ int DecoderSession_finish_ok(DecoderSession *d)
 			}
 		}
 	}
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+	/* Delete output file when fuzzing */
+	if(0 != d->fout && d->fout != stdout)
+		flac_unlink(d->outfilename);
+#endif
 	return ok? 0 : 1;
 }