gh-146250: Fix memory leak of re-initialization of SyntaxError

Added `Py_XDECREF`/`Py_XSETREF` to avoid a memory leak when calling
`SyntaxError.__init__`.

Author: bkap123

Lib/test/test_exceptions.py

@@ -1954,6 +1954,12 @@ def test_exec_set_nomemory_hang(self):
         self.assertGreater(len(output), 0)  # At minimum, should not hang
         self.assertIn(b"MemoryError", output)
 
+    # gh-146250: memory leak with re-initialization of SyntaxError
+    def test_syntax_error_memory_leak(self):
+        # Test crashes with ASan
+        e = SyntaxError("msg", ("file.py", 1, 2, "txt", 2, 3))
+        e.__init__("new_msg", ("new_file.py", 2, 3, "new_txt", 3, 4))
+
 
 class NameErrorTests(unittest.TestCase):
     def test_name_error_has_name(self):

Objects/exceptions.c

@@ -2802,8 +2802,14 @@ SyntaxError_init(PyObject *op, PyObject *args, PyObject *kwds)
             return -1;
         }
 
-        self->end_lineno = NULL;
-        self->end_offset = NULL;
+        Py_XDECREF(self->filename);
+        Py_XDECREF(self->lineno);
+        Py_XDECREF(self->offset);
+        Py_XDECREF(self->text);
+        Py_XSETREF(self->end_lineno, NULL);
+        Py_XSETREF(self->end_offset, NULL);
+        Py_XDECREF(self->metadata);
+
         if (!PyArg_ParseTuple(info, "OOOO|OOO",
                               &self->filename, &self->lineno,
                               &self->offset, &self->text,