Fix crash when attempting to parse a JSON document that contains an extension field for the wrong type of message.

I.e., if foo.extn is an extension for message foo.Foo, attempting to parse the document

  {"[foo.extn]": 4}

as a foo.Bar would crash. This CL causes the parser to return an error instead.

PiperOrigin-RevId: 501299336

src/google/protobuf/json/internal/parser.cc

@@ -1181,6 +1181,14 @@ absl::Status ParseField(JsonLexer& lex, const Desc<Traits>& desc,
   if (absl::StartsWith(name, "[") && absl::EndsWith(name, "]")) {
     absl::string_view extn_name = name.substr(1, name.size() - 2);
     field = Traits::ExtensionByName(desc, extn_name);
+
+    auto correct_type_name = Traits::TypeName(desc);
+    if (Traits::TypeName(Traits::ContainingType(*field)) != correct_type_name) {
+      return lex.Invalid(absl::StrFormat(
+          "'%s' is a known extension name, but is not an extenion "
+          "of '%s' as expected",
+          extn_name, correct_type_name));
+    }
   } else {
     field = Traits::FieldByName(desc, name);
   }

src/google/protobuf/json/json_test.cc

@@ -1042,6 +1042,11 @@ TEST_P(JsonTest, Extensions) {
           R"("[protobuf_unittest.TestMixedFieldsAndExtensions.c]":42,)"
           R"("b":[1,2,3],)"
           R"("[protobuf_unittest.TestMixedFieldsAndExtensions.d]":[1,1,2,3,5,8,13]})"));
+
+  auto m2 = ToProto<protobuf_unittest::TestAllTypes>(R"json({
+    "[protobuf_unittest.TestMixedFieldsAndExtensions.c]": 42
+  })json");
+  EXPECT_THAT(m2, StatusIs(absl::StatusCode::kInvalidArgument));
 }
 
 // Parsing does NOT work like MergeFrom: existing repeated field values are