bpo-29873: fix INCREF for possible NULL value in element_getattr() (GH-757)

Modules/_elementtree.c

@@ -1542,7 +1542,7 @@ element_getattr(ElementObject* self, char* name)
         return res;
     } else if (strcmp(name, "text") == 0) {
         res = element_get_text(self);
-        Py_INCREF(res);
+        Py_XINCREF(res);
         return res;
     }