[3.7] bpo-39831: Fix a reference leak in PyErr_WarnEx(). (GH-18750). (GH-18765)

(cherry picked from commit 2d2f855)

Author: serhiy-storchaka

Python/_warnings.c

@@ -710,7 +710,7 @@ setup_context(Py_ssize_t stack_level, PyObject **filename, int *lineno,
 
         *registry = PyDict_New();
         if (*registry == NULL)
-            goto handle_error;
+            return 0;
 
          rc = PyDict_SetItemString(globals, "__warningregistry__", *registry);
          if (rc < 0)
@@ -802,7 +802,6 @@ setup_context(Py_ssize_t stack_level, PyObject **filename, int *lineno,
        dangling reference. */
     Py_XDECREF(*registry);
     Py_XDECREF(*module);
-    Py_XDECREF(*filename);
     return 0;
 }