gh-105375: Improve errnomodule error handling (#105590)

Bail immediately if an exception is set, to prevent exceptions from
being overwritten.

Author: erlend-aasland

Misc/NEWS.d/next/Library/2023-06-09-21-04-39.gh-issue-105375.bTcqS9.rst

@@ -0,0 +1 @@
+Fix bugs in :mod:`pickle` where exceptions could be overwritten.

Modules/errnomodule.c

@@ -81,10 +81,12 @@ _add_errcode(PyObject *module_dict, PyObject *error_dict, const char *name_str,
 static int
 errno_exec(PyObject *module)
 {
-    PyObject *module_dict = PyModule_GetDict(module);
+    PyObject *module_dict = PyModule_GetDict(module);  // Borrowed ref.
+    if (module_dict == NULL) {
+        return -1;
+    }
     PyObject *error_dict = PyDict_New();
-    if (!module_dict || !error_dict) {
-        Py_XDECREF(error_dict);
+    if (error_dict == NULL) {
         return -1;
     }
     if (PyDict_SetItemString(module_dict, "errorcode", error_dict) < 0) {