[CrOS MultiDevice] Eliminate chrome://proximity-auth crash.

The crash occurs when trying to dereference a null pointer. This fix
simply returns early if debug information is not present.

Bug: 936635
Change-Id: I034ba93d2fb027dd379f684393c1368b11a73e7c
Reviewed-on: https://chromium-review.googlesource.com/c/1493513
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jeremy Klein <jlklein@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636284}

chromeos/components/multidevice/debug_webui/proximity_auth_webui_handler.cc

@@ -527,6 +527,10 @@ void ProximityAuthWebUIHandler::OnFindEligibleDevices(
 
 void ProximityAuthWebUIHandler::OnGetDebugInfo(
     device_sync::mojom::DebugInfoPtr debug_info_ptr) {
+  // If enrollment is not yet complete, no debug information is available.
+  if (!debug_info_ptr)
+    return;
+
   if (enrollment_update_waiting_for_debug_info_) {
     enrollment_update_waiting_for_debug_info_ = false;
     NotifyOnEnrollmentFinished(