Assert that dereferenced pointer is never nullptr

It can't be, since then 'o' (which is never nullptr) would be different,
and we'd return in the previous line.

Since we test for d->popup a few lines above, help clang static analyzer
to not report this with hash
27ff831d79d1779825fd964239cbefe7.

Change-Id: I9f0b14bd88418b897cee2d7a63e8a10fc4a0b01c
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit f6c9dec610238bae0d2d61e733d4c29ecf57aa9d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/util/qcompleter.cpp

@@ -1329,6 +1329,7 @@ bool QCompleter::eventFilter(QObject *o, QEvent *e)
     if (o != d->popup)
         return QObject::eventFilter(o, e);
 
+    Q_ASSERT(d->popup);
     switch (e->type()) {
     case QEvent::KeyPress: {
         QKeyEvent *ke = static_cast<QKeyEvent *>(e);