faulthandler: fix unregister() if it is called before register()

Fix a crash: don't read from NULL.

Doc/library/faulthandler.rst

@@ -97,7 +97,8 @@ Dump the traceback on a user signal
 .. function:: unregister(signum)
 
    Unregister a user signal: uninstall the handler of the *signum* signal
-   installed by :func:`register`.
+   installed by :func:`register`. Return ``True`` if the signal was registered,
+   ``False`` otherwise.
 
    Not available on Windows.
 

Modules/faulthandler.c

@@ -694,6 +694,9 @@ faulthandler_unregister_py(PyObject *self, PyObject *args)
     if (!check_signum(signum))
         return NULL;
 
+    if (user_signals == NULL)
+        Py_RETURN_FALSE;
+
     user = &user_signals[signum];
     change = faulthandler_unregister(user, signum);
     return PyBool_FromLong(change);