[3.15] gh-157892: Fix error check on _Py_hashtable_set() in intern_static() (GH-157898) (GH-158301)

_Py_hashtable_set() returns -1 on failure, so the "< -1" check could never be true and the Py_FatalError() call was unreachable.
(cherry picked from commit a4eb936)

Co-authored-by: Rishi Raj <180109771+rishiraj38@users.noreply.github.com>

Author: miss-islington

Objects/unicodeobject.c

@@ -14181,7 +14181,7 @@ intern_static(PyInterpreterState *interp, PyObject *s /* stolen */)
         return Py_NewRef(r);
     }
 
-    if (_Py_hashtable_set(INTERNED_STRINGS, s, s) < -1) {
+    if (_Py_hashtable_set(INTERNED_STRINGS, s, s) < 0) {
         Py_FatalError("failed to intern static string");
     }