catch nullptr effects when deleting shadows

the shadow can be deleted deferred from an update
there's a slight chance, to be eg. triggered by clients
blocking compositing, that the compositor is suspended in the same
cycle and the effects pointer (and scene and context) thus gone

CCBUG: 356938
FIXED-IN: 5.6
REVIEW: 126441

scene_opengl.cpp

@@ -2195,9 +2195,11 @@ SceneOpenGLShadow::SceneOpenGLShadow(Toplevel *toplevel)
 
 SceneOpenGLShadow::~SceneOpenGLShadow()
 {
+    if (effects) {
         effects->makeOpenGLContextCurrent();
         DecorationShadowTextureCache::instance().unregister(this);
         m_texture.reset();
+    }
 }
 
 void SceneOpenGLShadow::buildQuads()