Fix #35625, crash with NULL pointer after node render. Happens when node group node_tree pointer is NULL, then the group node doesn not produce any execdata and needs to check this accordingly in the
freeexec callback.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57247 954f8c5b-7b00-dc11-b283-0030488c597c