Fix a crash in Geolocation Controller.

Fixes a crash when there is no visible item.

Bug: 819200
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Icc97de304a7720d7866b309cc436eb2938c833ce
Reviewed-on: https://chromium-review.googlesource.com/1170778
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584399}

ios/chrome/browser/geolocation/omnibox_geolocation_controller.mm

@@ -457,7 +457,8 @@ const char* const kGeolocationAuthorizationActionNewUser =
     web::NavigationManager* navigationManager =
         tab.webState->GetNavigationManager();
     web::NavigationItem* item = navigationManager->GetVisibleItem();
-    if ([self addLocationToNavigationItem:item browserState:tab.browserState]) {
+    if (item &&
+        [self addLocationToNavigationItem:item browserState:tab.browserState]) {
       navigationManager->Reload(web::ReloadType::NORMAL,
                                 false /* check_for_repost */);
     }