Fix typo in QAbstractItemModelTester

When recursively checking children in QAbstractItemModelTester,
the currentDepth of recursion was incorrectly increased.
This patch fixes it.

Pick-to: 6.2 5.15
Change-Id: Ia79508f358e2fcb6b90780a79baec053522e871c
Reviewed-by: David Faure <david.faure@kdab.com>

src/testlib/qabstractitemmodeltester.cpp

@@ -570,7 +570,7 @@ void QAbstractItemModelTesterPrivate::checkChildren(const QModelIndex &parent, i
 
             // recursively go down the children
             if (model->hasChildren(index) && currentDepth < 10)
-                checkChildren(index, ++currentDepth);
+                checkChildren(index, currentDepth + 1);
 
             // make sure that after testing the children that the index doesn't change.
             QModelIndex newerIndex = model->index(r, c, parent);