[dwm] Bug: trying to toggletag when there is no focused window => dwm crashes

Good evening||morning,

I noticed a bug in dwm 5.1 -- the description in the subject. The reason
is using sel->tags in function toggletag before checking whether sel is
not NULL.

The solution is obvious ("if (!sel) return;" before "...sel->tags...").
Patch attached.

Best regards
*j

-- 
Jan Kaliszewski (zuo)