Initialize variable that is detected as 'maybe uninitialized'.

With gcc (tested versions: 11.3.0, 12.2.0) compiled with asan, the
`type_id` variable is warned about maybe being used uninitialized in
the later call:

```
 ptr = ParseFieldMaybeLazily(static_cast<uint64_t(type_id) * 8 + 2, ptr,
                             extendee, metadata, ctx);
```

This change initializes the variable to a defined default.

PiperOrigin-RevId: 507883964

src/google/protobuf/extension_set_inl.h

@@ -206,7 +206,7 @@ const char* ExtensionSet::ParseMessageSetItemTmpl(
     const char* ptr, const Msg* extendee, internal::InternalMetadata* metadata,
     internal::ParseContext* ctx) {
   std::string payload;
-  uint32_t type_id;
+  uint32_t type_id = 0;
   enum class State { kNoTag, kHasType, kHasPayload, kDone };
   State state = State::kNoTag;