Add simple fuzz target for upb

tests/file_descriptor_parsenew_fuzzer.cc

@@ -0,0 +1,16 @@
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+
+#include "google/protobuf/descriptor.upb.h"
+#include "upb/def.h"
+#include "upb/msg.h"
+#include "upb/upb.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  upb_strview strview =
+      upb_strview_make(reinterpret_cast<const char*>(data), size);
+  upb::Arena arena;
+  google_protobuf_FileDescriptorProto_parsenew(strview, arena.ptr());
+  return 0;
+}