[QDomNodePrivate] delete unnecessary pointer check from the statement

- prev pointer never can be null here, so the check can be deleted.

Ammends 948599e

Pick-to: 6.8 6.9
Change-Id: Ie194f5f0432f6da5f6471328193112c970f623b6
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/xml/dom/qdom.cpp

@@ -1445,7 +1445,7 @@ void QDomNodePrivate::saveSubTree(const QDomNodePrivate *n, QTextStream &s,
             const QDomNodePrivate *prev = root;
             root = root->next;
             // Close QDomElementPrivate groups
-            while (!root && prev && (layerDepth > 0)) {
+            while (!root && (layerDepth > 0)) {
                 root = prev->parent();
                 layerDepth --;
                 root->afterSave(s, layerDepth + branchDepth, indent);