gh-145989: Fix double Py_DECREF in pyexpat ExternalEntityParserCreate

Remove redundant Py_DECREF(self) from three error paths in
ExternalEntityParserCreate_impl(). The dealloc of new_parser
already decrements the parent reference via Py_CLEAR(self->parent).

Use Py_NewRef() for the parent assignment for clarity.

Author: 0xCyberstan

Modules/pyexpat.c

@@ -1086,7 +1086,6 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
     // The new subparser will make use of the parent XML_Parser inside of Expat.
     // So we need to take subparsers into account with the reference counting
     // of their parent parser.
-    Py_INCREF(self);
 
     new_parser->buffer_size = self->buffer_size;
     new_parser->buffer_used = 0;
@@ -1097,21 +1096,19 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
     new_parser->ns_prefixes = self->ns_prefixes;
     new_parser->itself = XML_ExternalEntityParserCreate(self->itself, context,
                                                         encoding);
-    new_parser->parent = (PyObject *)self;
+    new_parser->parent = Py_NewRef(self);
     new_parser->handlers = 0;
     new_parser->intern = Py_XNewRef(self->intern);
 
     if (self->buffer != NULL) {
         new_parser->buffer = PyMem_Malloc(new_parser->buffer_size);
         if (new_parser->buffer == NULL) {
             Py_DECREF(new_parser);
-            Py_DECREF(self);
             return PyErr_NoMemory();
         }
     }
     if (!new_parser->itself) {
         Py_DECREF(new_parser);
-        Py_DECREF(self);
         return PyErr_NoMemory();
     }
 
@@ -1125,7 +1122,6 @@ pyexpat_xmlparser_ExternalEntityParserCreate_impl(xmlparseobject *self,
     new_parser->handlers = PyMem_New(PyObject *, i);
     if (!new_parser->handlers) {
         Py_DECREF(new_parser);
-        Py_DECREF(self);
         return PyErr_NoMemory();
     }
     clear_handlers(new_parser, 1);