Make sure that *m_twinSpacer is iniitialized in panelspacer

This fixes a segfault of panelspacer on musl and 32bit glibc
systems.

BUG: 422914

applets/panelspacer/plugin/panelspacer.h

@@ -73,5 +73,5 @@ Q_SIGNALS:
     void twinSpacerChanged();
 
 private:
-    PlasmaQuick::AppletQuickItem *m_twinSpacer;
+    PlasmaQuick::AppletQuickItem *m_twinSpacer = nullptr;
 };