Check for existence of current blocking page before forwarding commands

This will prevent crashes in cases where the current blocking page
reference has been released. Underlying issue will be fixed in a
follow-up CL.

Bug: 1044768,1045038
Change-Id: Ieda79f321a3f933369a74e539267ead95106d6e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015669Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735648}

ios/chrome/browser/ssl/ios_ssl_error_tab_helper.mm

@@ -95,8 +95,10 @@ void IOSSSLErrorTabHelper::OnBlockingPageCommand(
   if (!message.GetString("command", &command)) {
     DLOG(WARNING) << "JS message parameter not found: command";
   } else {
-    blocking_page_for_currently_committed_navigation_->HandleScriptCommand(
-        message, url, user_is_interacting, sender_frame);
+    if (blocking_page_for_currently_committed_navigation_) {
+      blocking_page_for_currently_committed_navigation_->HandleScriptCommand(
+          message, url, user_is_interacting, sender_frame);
+    }
   }
 }