effectstackview2.cpp: check for null pointer

Fixes Coverity CID 709290: Dereference after null check (FORWARD_NULL)
Comparing "this->m_clipref" to null implies that "this->m_clipref" might be null.
717    else if (m_clipref) {
...
Passing null variable "this->m_clipref" to function "AbstractClipItem::info() const", which dereferences it. [show details]
733        info = m_clipref->info();