highlighter: handle color names when converting into msdos codes

the highlighter code tries to convert the RGB colors into appropriate
color codes for the MSDOS palette. Unfortunately, it does not consider
color names and tries to split those into a list of 3 RGB codes. This
failes for names shorter 6 characters, causing a list index out of
bounds error.

Fix this by making sure, that the color code should start with '#' and
in case it does not, assume it is a color name and simple return the
name in that case.

closes #2350

autoload/airline/highlighter.vim

@@ -18,6 +18,9 @@ function! s:gui2cui(rgb, fallback) abort
     return a:fallback
   elseif match(a:rgb, '^\%(NONE\|[fb]g\)$') > -1
     return a:rgb
+  elseif a:rgb[0] !~ '#'
+    " a:rgb contains colorname
+    return a:rgb
   endif
   let rgb = map(split(a:rgb[1:], '..\zs'), '0 + ("0x".v:val)')
   return airline#msdos#round_msdos_colors(rgb)