Check that context exists before trying to pop context.

On some linux drivers (such as nvidia version 384.111 and 387.34) setting the
context to zero causes a segfault so check before setting in cases where the
destruction may be trying to pop to a zero context.

(cherry picked from commit a7988f5c)

Rendering/OpenGL2/vtkXOpenGLRenderWindow.cxx

@@ -1151,7 +1151,7 @@ void vtkXOpenGLRenderWindow::PopContext()
   GLXContext current = glXGetCurrentContext();
   GLXContext target = static_cast<GLXContext>(this->ContextStack.top());
   this->ContextStack.pop();
-  if (target != current)
+  if (target && target != current)
   {
     glXMakeCurrent(this->DisplayStack.top(),
       this->DrawableStack.top(),