widget: Fix a memory leak in set_colors
widget: Fix a memory leak in set_colors

https://bugzilla.gnome.org/show_bug.cgi?id=768994

thestinger/vte-ng,saitoha/vte-sixel,thestinger/vte-ng,saitoha/vte-sixel,thestinger/vte-ng,saitoha/vte-sixel