Avoid accessing freed nodes in cancel_presel
Avoid accessing freed nodes in cancel_presel

Since cancel_presel is called by destroy_tree, we have to be careful not
to access any node besides n.

nfnty/bspwm,nfnty/bspwm,JBouron/bspwm,JBouron/bspwm,JBouron/bspwm,medisun/bspwm,medisun/bspwm,medisun/bspwm,nfnty/bspwm,baskerville/bspwm,baskerville/bspwm