Fix uninitialized return value from constraint operators

The utility function edit_constraint_invoke_properties could return
false without setting the return value, causing the operators invoke
function to return an uninitialized flag.

source/blender/editors/object/object_constraint.cc

@@ -782,6 +782,9 @@ static bool edit_constraint_invoke_properties(bContext *C,
     }
   }
 
+  if (r_retval != nullptr) {
+    *r_retval = OPERATOR_CANCELLED;
+  }
   return false;
 }