Bug 1659825 - Null check gWindowContexts in WindowContext::Discard(). r=nika

If we're after shutdown, then this variable will have been cleared.
This is the remaining use that was failing to do a null check, and
it was manifesting as a very low-volume crash.

Differential Revision: https://phabricator.services.mozilla.com/D87503

docshell/base/WindowContext.cpp

@@ -304,7 +304,9 @@ void WindowContext::Discard() {
   }
 
   mIsDiscarded = true;
-  gWindowContexts->Remove(InnerWindowId());
+  if (gWindowContexts) {
+    gWindowContexts->Remove(InnerWindowId());
+  }
   mBrowsingContext->UnregisterWindowContext(this);
   Group()->Unregister(this);
 }