i3bar: ensure get_buffer does not leak memory
i3bar: ensure get_buffer does not leak memory

This fixes an AddressSanitizer warning which recently popped up.

related to #2907

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