effectstackview2.cpp: check for null

Fixes Coverity CID 709289: Explicit null dereferenced (FORWARD_NULL)
Assigning: "effectToMove" = 0.
790    CollapsibleEffect *effectToMove = NULL;
...
Passing null variable "effectToMove" to function "CollapsibleGroup::addGroupEffect(CollapsibleEffect *)", which dereferences it. [show details]
805    group->addGroupEffect(effectToMove);