Fix segment fault for UPB Pyhon 'in' method of empty repeated extensions

PiperOrigin-RevId: 741359924

Author: anandolee

python/extension_dict.c

@@ -101,6 +101,7 @@ static int PyUpb_ExtensionDict_Contains(PyObject* _self, PyObject* key) {
   if (!msg) return 0;
   if (upb_FieldDef_IsRepeated(f)) {
     upb_MessageValue val = upb_Message_GetFieldByDef(msg, f);
+    if (!val.array_val) return 0;
     return upb_Array_Size(val.array_val) > 0;
   } else {
     return upb_Message_HasFieldByDef(msg, f);

python/google/protobuf/internal/reflection_test.py

@@ -1902,6 +1902,13 @@ def testMergeFromExtensionsNestedMessage(self):
     self.assertEqual(222, ext2[1].bb)
     self.assertEqual(333, ext2[2].bb)
 
+  def testRepeatedExtensionsIn(self):
+    msg = more_extensions_pb2.ExtendedMessage()
+    ext = more_extensions_pb2.repeated_int_extension
+    self.assertNotIn(ext, msg.Extensions)
+    msg.Extensions[ext].append(123)
+    self.assertIn(ext, msg.Extensions)
+
   def testCopyFromBadType(self):
     proto1 = unittest_pb2.TestAllTypes()
     proto2 = unittest_pb2.TestAllExtensions()