Added a pull request to fix a colorset usage bug in FvwmIconMan

Hi all,

(I'm new here, so please pardon me if this is the wrong forum)  As per the
developer's guide, I'm emailing the list to let you know of a bug fix made
in this pull request: https://github.com/fvwmorg/fvwm3/pull/6

This bug has been present for a while, but I can see that development to
FVWM2 is closed, so I'm submitting this against 3.  It's a pretty trivial
fix.

Thanks for your attention,
-Matt