[iOS] Fix a potential null dereference in LocationBarMediator.

Adds a check before dereferencing self.webState.

Bug: 1055334
Change-Id: Iaa046b5975d72cb7632783289ed9fdb9088a15f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080418
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#745449}

ios/chrome/browser/ui/location_bar/location_bar_mediator.mm

@@ -341,6 +341,10 @@
   if (self.webContentAreaShowingOverlay)
     return NO;
 
+  if (!self.webState) {
+    return NO;
+  }
+
   const GURL& URL = self.webState->GetLastCommittedURL();
   return URL.is_valid() && !web::GetWebClient()->IsAppSpecificURL(URL);
 }