patch 9.0.1209: getting interface member does not always work

Problem:    Getting interface member does not always work.
Solution:   Convert the index on the interface to the index on the object.
            (closes #11825)

src/vim9expr.c

@@ -364,6 +364,8 @@ compile_class_object_index(cctx_T *cctx, char_u **arg, type_T *type)
 		}
 
 		*arg = name_end;
+		if (cl->class_flags & CLASS_INTERFACE)
+		    return generate_GET_ITF_MEMBER(cctx, cl, i, m->ocm_type);
 		return generate_GET_OBJ_MEMBER(cctx, i, m->ocm_type);
 	    }
 	}