faulthandler: fix unregister() if it is called before register()
Fix a crash: don't read from NULL.
diff --git a/Doc/library/faulthandler.rst b/Doc/library/faulthandler.rst
index 106da45..c0b6625 100644
--- a/Doc/library/faulthandler.rst
+++ b/Doc/library/faulthandler.rst
@@ -97,7 +97,8 @@
.. 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.