fix and update libfuzzer custom mutator

custom_mutators/libfuzzer/FuzzerMain.cpp

@@ -0,0 +1,26 @@
+//===- FuzzerMain.cpp - main() function and flags -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// main() and flags.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#include "FuzzerPlatform.h"
+
+extern "C" {
+
+// This function should be defined by the user.
+int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+
+}  // extern "C"
+
+ATTRIBUTE_INTERFACE int main(int argc, char **argv) {
+
+  return fuzzer::FuzzerDriver(&argc, &argv, LLVMFuzzerTestOneInput);
+
+}
+