x11-wm/fluxbox: fix integer underflow in startup phase

See also
http://git.fluxbox.org/fluxbox.git/commit/?id=7b8fd2d81ad80a73564fc9fbb779f47568f12652

PR: 197150
Submitted by: vikashb@where-ever.za.net
Approved by: portmgr runtime fix blanket