Fix null-ptr crash in fallback cursor

This crash happens because the hit test returns a null node. In that
case it's safe to just return without doing anything. Not sure how to
set this case up in a test though.

Bug: 982360
Change-Id: I20d32e2704543ee816ac17c31a51957b30366309
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706355
Commit-Queue: David Bokan <bokan@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Auto-Submit: David Bokan <bokan@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678296}

third_party/blink/renderer/core/input/fallback_cursor_event_manager.cc

@@ -392,6 +392,8 @@ void FallbackCursorEventManager::HandleMousePressEvent(const WebMouseEvent& e) {
   HitTestResult hit_test_result =
       HitTest(root_frame_->GetDocument()->GetLayoutView(), location);
   Node* node = hit_test_result.InnerNode();
+  if (!node)
+    return;
 
   // Click on input boxes or media node should hide the cursor.
   if (HasEditableStyle(*node) || node->IsMediaElement()) {