Fix wrong call to free
Fix wrong call to free

To confirm, assign n to a constant value and try to use the
append_layout command.

Without the change i3 crashes.

Airblader/i3,Airblader/i3,i3/i3,Airblader/i3-original,Airblader/i3-original,cornerman/i3,Airblader/i3-original,Airblader/i3-original,cornerman/i3,Airblader/i3-original,cornerman/i3,i3/i3,cornerman/i3,stapelberg/i3,stapelberg/i3,Airblader/i3,i3/i3,Airblader/i3,i3/i3,stapelberg/i3,stapelberg/i3