Fix several reference counting bugs in pyexpat.c. (GH-9955)

(cherry picked from commit 68def052)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Modules/pyexpat.c

@@ -398,8 +398,10 @@ string_intern(xmlparseobject *self, const char* str)
     if (!value) {
         if (PyDict_SetItem(self->intern, result, result) == 0)
             return result;
-        else
+        else {
+            Py_DECREF(result);
             return NULL;
+        }
     }
     Py_INCREF(value);
     Py_DECREF(result);
@@ -547,6 +549,7 @@ my_StartElementHandler(void *userData,
                 flag_error(self);
                 Py_DECREF(n);
                 Py_DECREF(v);
+                Py_DECREF(container);
                 return;
             }
             else {
@@ -555,12 +558,14 @@ my_StartElementHandler(void *userData,
             }
         }
         args = string_intern(self, name);
-        if (args != NULL)
-            args = Py_BuildValue("(NN)", args, container);
         if (args == NULL) {
             Py_DECREF(container);
             return;
         }
+        args = Py_BuildValue("(NN)", args, container);
+        if (args == NULL) {
+            return;
+        }
         /* Container is now a borrowed reference; ignore it. */
         self->in_callback = 1;
         rv = call_with_frame(getcode(StartElement, "StartElement", __LINE__),
@@ -742,7 +747,6 @@ my_ElementDeclHandler(void *userData,
         }
         args = Py_BuildValue("NN", nameobj, modelobj);
         if (args == NULL) {
-            Py_DECREF(modelobj);
             flag_error(self);
             goto finally;
         }