Bug 1781169: Ensure that the cached start/end containers contain characters...

Bug 1781169: Ensure that the cached start/end containers contain characters before returning them in DocAccessibleParent::SelectionRanges. r=eeejay

In bug 1772170, I added code to ensure that the cached offsets are valid.
However, this didn't account for the possibility that one of the containers contained 0 characters.
In that case, offset 0 is itself invalid.
If there were also no children, this might in GetChildAtOffset being called and returning null, causing a crash when retrieving selection.

Differential Revision: https://phabricator.services.mozilla.com/D154964

accessible/ipc/DocAccessibleParent.cpp

@@ -1206,11 +1206,12 @@ void DocAccessibleParent::SelectionRanges(nsTArray<TextRange>* aRanges) const {
       continue;
     }
     uint32_t startCount = startAcc->CharacterCount();
-    if (data.StartOffset() > static_cast<int32_t>(startCount)) {
+    if (startCount == 0 ||
+        data.StartOffset() > static_cast<int32_t>(startCount)) {
       continue;
     }
     uint32_t endCount = endAcc->CharacterCount();
-    if (data.EndOffset() > static_cast<int32_t>(endCount)) {
+    if (endCount == 0 || data.EndOffset() > static_cast<int32_t>(endCount)) {
       continue;
     }
     aRanges->AppendElement(TextRange(const_cast<DocAccessibleParent*>(this),