[wmii] BUGFIX? Fix crash when applying tag /./ and switching to another tag

Hi,

How to reproduce:
1) Start wmii
2) Open up one window (terminal)
3) Run wmiir xwrite /client/sel/tags /./
4) Switch to tag 2 using Mod+2
5) wmii crashes

Proposed patch is attached, please review if this is the correct solution. In
my understanding, only floating windows were checked, but you can apply /./ to
all windows, so we need to check (and apply_tags) all windows. Feel free to
enlighten me, if I'm wrong.

Best regards,
Michael