[3.12] gh-105375: Improve error handling in _elementtree (GH-105591) (#105600)

Fix bugs where exceptions could end up being overwritten.
(cherry picked from commit 00b599a)

Co-authored-by: Erlend E. Aasland <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2023-06-09-21-25-14.gh-issue-105375.95g1eI.rst

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

Modules/_elementtree.c

@@ -3261,10 +3261,14 @@ expat_start_handler(XMLParserObject* self, const XML_Char* tag_in,
         }
         while (attrib_in[0] && attrib_in[1]) {
             PyObject* key = makeuniversal(self, attrib_in[0]);
+            if (key == NULL) {
+                Py_DECREF(attrib);
+                Py_DECREF(tag);
+                return;
+            }
             PyObject* value = PyUnicode_DecodeUTF8(attrib_in[1], strlen(attrib_in[1]), "strict");
-            if (!key || !value) {
-                Py_XDECREF(value);
-                Py_XDECREF(key);
+            if (value == NULL) {
+                Py_DECREF(key);
                 Py_DECREF(attrib);
                 Py_DECREF(tag);
                 return;