#7 fbxkb crashes when layouts count is decreased

[Aleks-Daniel Jakimenko-Aleksejev]

How to replicate:
1) run:
setxkbmap -option grp:alt_shift_toggle 'us,de'
2) now switch your layout to "de" with alt+shift
3) run:
setxkbmap 'us'
4) enjoy fbxkb crashing with this error:
ERROR:fbxkb.c:335:read_kbd_description: assertion failed: (cur_group < ngroups)