Fix crash in CaptivePortalTabReloader

This CL adds a null check for the last committed NavigationEntry to fix
a crash in CaptivePortalTabReloader.

Bug: 857002
Change-Id: Ia26bd420cbee80958b3a74937ade9efb764ed03a
Reviewed-on: https://chromium-review.googlesource.com/1146657
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577919}

chrome/browser/captive_portal/captive_portal_tab_reloader.cc

@@ -266,8 +266,10 @@ void CaptivePortalTabReloader::ReloadTabIfNeeded() {
 
 void CaptivePortalTabReloader::ReloadTab() {
   content::NavigationController* controller = &web_contents_->GetController();
-  if (!controller->GetLastCommittedEntry()->GetHasPostData())
+  if (controller->GetLastCommittedEntry() &&
+      !controller->GetLastCommittedEntry()->GetHasPostData()) {
     controller->Reload(content::ReloadType::NORMAL, true);
+  }
 }
 
 void CaptivePortalTabReloader::MaybeOpenCaptivePortalLoginTab() {