[iOS][AF] Fix crash when no suggestions available

What was happening?
When the keyboard was hidden, te form suggestion controller was being
reset. At the same time, the form observer had triggered a suggestions
look up, causing call to an unexistent block.

Bug: 875177
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ie42fc8588fd368a24779134ad9370114995fefaf
Reviewed-on: https://chromium-review.googlesource.com/1183181Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584728}

ios/chrome/browser/autofill/form_suggestion_controller.mm

@@ -265,7 +265,10 @@ AutofillSuggestionState::AutofillSuggestionState(
 }
 
 - (void)onNoSuggestionsAvailable {
-  DCHECK(accessoryViewUpdateBlock_);
+  // Check the update block hasn't been reset while waiting for suggestions.
+  if (!accessoryViewUpdateBlock_) {
+    return;
+  }
   BOOL isManualFillEnabled =
       base::FeatureList::IsEnabled(autofill::features::kAutofillManualFallback);
   if (isManualFillEnabled) {