Assert that QQmlDelegateModel's count can't be less than zero

I was running into a crash (it didn't reproduce with a minimal example,
and I'm still not sure exactly what caused it) related to models and
views, and saw that there m_count was negative, which shouldn't be
possible. Adding this new assertion allowed me to find the issue in my
code.

Pick-to: 5.15 6.2 6.3
Change-Id: Ie467e749dbf95799618bd04591e50b0038cf3399
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/qmlmodels/qqmldelegatemodel.cpp

@@ -1746,6 +1746,7 @@ void QQmlDelegateModel::_q_itemsRemoved(int index, int count)
         return;
 
     d->m_count -= count;
+    Q_ASSERT(d->m_count >= 0);
     const QList<QQmlDelegateModelItem *> cache = d->m_cache;
     //Prevents items being deleted in remove loop
     for (QQmlDelegateModelItem *item : cache)