Bug 1493403: AccGroupInfo::TotalItemCount: Don't assume a container has children. r=eeejay

The code does check for the case where a container has no children.
However, before it does so, it tries to check whether it needs to skip a text leaf child.
This check previously assumed there was a child to check, which crashed for a container with no children.

Differential Revision: https://phabricator.services.mozilla.com/D6588

--HG--
extra : moz-landing-system : lando

accessible/base/AccGroupInfo.cpp

@@ -233,7 +233,7 @@ AccGroupInfo::TotalItemCount(Accessible* aContainer, bool* aIsHierarchical)
       Accessible* childItem = AccGroupInfo::FirstItemOf(aContainer);
       if (!childItem) {
         childItem = aContainer->FirstChild();
-        if (childItem->IsTextLeaf()) {
+        if (childItem && childItem->IsTextLeaf()) {
           // First child can be a text leaf, check its sibling for an item.
           childItem = childItem->NextSibling();
         }