FbTk/StringUtil.cc: Fix out-of-range memory access.
FbTk/StringUtil.cc: Fix out-of-range memory access.

if pos is not npos, it will always be less than filename.size().
However, the access later is only safe if there is a character
after pos, which would require pos + 1 to be less than filename.size.

Arkq/fluxbox,olivergondza/fluxbox,Arkq/fluxbox,naimdjon/fluxbox,lukoko/fluxbox,luebking/fluxbox,naimdjon/fluxbox,luebking/fluxbox,olivergondza/fluxbox,lukoko/fluxbox,lukoko/fluxbox,olivergondza/fluxbox,naimdjon/fluxbox,naimdjon/fluxbox,Arkq/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,luebking/fluxbox,Arkq/fluxbox,lukoko/fluxbox,naimdjon/fluxbox,lukoko/fluxbox,luebking/fluxbox,olivergondza/fluxbox,luebking/fluxbox