plugins/slidingpopups: Fix a crash on compositing restart

The sliding popups effect can be unloaded while there's still an active
animation for a closed window.

If that happens, the EffectsHandler::windowDeleted signal may be emitted
when the EffectWindowDeletedRef objects are destroyed. It happens after
the SlidingPopupsEffect destructor.

The sliding popups effect also has an EffectsHandler::windowDeleted
slot.

So, the problem is that m_animationsData gets destroyed first, then
m_animations is destroyed next. When m_animations is destroyed, the
deleted references will be released and EffectsHandler::windowDeleted
will be emitted. SlidingPopupsEffect::slotWindowDeleted will access
m_animationsData whose memory has been just released.

In order to prevent crashing, this change ensures that animations are
canceled while m_animationsData is still valid.

src/plugins/slidingpopups/slidingpopups.cpp

@@ -82,6 +82,11 @@ SlidingPopupsEffect::~SlidingPopupsEffect()
     if (s_slideManager) {
         s_slideManagerRemoveTimer->start(1000);
     }
+
+    // Cancel animations here while both m_animations and m_animationsData are still valid.
+    // slotWindowDeleted may access m_animationsData when an animation is removed.
+    m_animations.clear();
+    m_animationsData.clear();
 }
 
 bool SlidingPopupsEffect::supported()