Avoid crashes if all favicons already processed

We have evidence of DownloadCurrentCandidateOrAskFaviconService()
crashing due to current_candidate() being null, at least until recent
fixes.

Temporarily and as a precaution, we return early in case the problematic
scenario is reproduced. We suspect this condition is not reached in
trunk, but it's a simple workaround to cherry-pick.

Bug: 828196
Change-Id: I41e2236f341a9ed9e18a950540f096e9de42df0b
Reviewed-on: https://chromium-review.googlesource.com/1002093Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549264}

components/favicon/core/favicon_handler.cc

@@ -458,6 +458,10 @@ void FaviconHandler::OnGotInitialHistoryDataAndIconURLCandidates() {
     return;
   }
 
+  if (!current_candidate()) {
+    return;
+  }
+
   if (!initial_history_result_expired_or_incomplete_ &&
       current_candidate()->icon_url == notification_icon_url_ &&
       current_candidate()->icon_type == notification_icon_type_) {