gh-128923: Use zero to indicate unassigned unique id (#128925)

In the free threading build, the per thread reference counting uses a
unique id for some objects to index into the local reference count
table. Use 0 instead of -1 to indicate that the id is not assigned. This
avoids bugs where zero-initialized heap type objects look like they have
a unique id assigned.

Objects/codeobject.c

@@ -1911,7 +1911,7 @@ code_dealloc(PyObject *self)
     Py_XDECREF(co->co_linetable);
     Py_XDECREF(co->co_exceptiontable);
 #ifdef Py_GIL_DISABLED
-    assert(co->_co_unique_id == -1);
+    assert(co->_co_unique_id == _Py_INVALID_UNIQUE_ID);
 #endif
     if (co->_co_cached != NULL) {
         Py_XDECREF(co->_co_cached->_co_code);