Bug 1680977 - Add a null-check.

aLink might be null in the parent process, and even though it's only
null when getting URIs via IPC (which should never hit this code path in
a correct execution), we don't trust the child process so this is safer.

Differential Revision: https://phabricator.services.mozilla.com/D99703

docshell/base/BaseHistory.cpp

@@ -80,7 +80,10 @@ void BaseHistory::RegisterVisitedCallback(nsIURI* aURI, Link* aLink) {
   }
 
   if (!CanStore(aURI)) {
-    return aLink->VisitedQueryFinished(/* visited = */ false);
+    if (aLink) {
+      aLink->VisitedQueryFinished(/* visited = */ false);
+    }
+    return;
   }
 
   // Obtain our array of observers for this URI.