fix: contains check segfaults on empty map (#20446) (#20904)

Fixes #19624

COPYBARA_INTEGRATE_REVIEW=#20446 from mjvankampen:fix/segfault-empty-map ea87203
PiperOrigin-RevId: 736519479

Co-authored-by: Mark Jan van Kampen <m.vankampen@freeday.ai>

Author: neilconway

python/google/protobuf/internal/well_known_types_test.py

@@ -683,6 +683,11 @@ def testDurationSub(self, old_time, time_delta, expected_sec, expected_nano):
 
 class StructTest(unittest.TestCase):
 
+  def testEmptyDict(self):
+    # in operator for empty initialized struct
+    msg = well_known_types_test_pb2.WKTMessage(optional_struct={})
+    self.assertNotIn('key', msg.optional_struct)
+
   def testStruct(self):
     struct = struct_pb2.Struct()
     self.assertIsInstance(struct, collections_abc.Mapping)

python/message.c

@@ -1103,6 +1103,7 @@ static PyObject* PyUpb_Message_Contains(PyObject* _self, PyObject* arg) {
       upb_Message* msg = PyUpb_Message_GetMsg(self);
       const upb_FieldDef* f = upb_MessageDef_FindFieldByName(msgdef, "fields");
       const upb_Map* map = upb_Message_GetFieldByDef(msg, f).map_val;
+      if (!map || upb_Map_Size(map) == 0) Py_RETURN_FALSE;
       const upb_MessageDef* entry_m = upb_FieldDef_MessageSubDef(f);
       const upb_FieldDef* key_f = upb_MessageDef_Field(entry_m, 0);
       upb_MessageValue u_key;