iOS Accessibility: implement accessibilityElements and check indexing

Fixes: QTBUG-70683
Change-Id: I122c67a5cee22363de5c8e45dc1c83e7760162fb
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/ios/quiview_accessibility.mm

@@ -101,6 +101,8 @@
 - (id)accessibilityElementAtIndex:(NSInteger)index
 {
     [self initAccessibility];
+    if (index >= [m_accessibleElements count])
+        return nil;
     return m_accessibleElements[index];
 }
 
@@ -110,4 +112,10 @@
     return [m_accessibleElements indexOfObject:element];
 }
 
+- (NSArray *)accessibilityElements
+{
+    [self initAccessibility];
+    return m_accessibleElements;
+}
+
 @end