account for empty vimrc with no colorscheme set.

autoload/airline.vim

@@ -21,6 +21,7 @@ function! airline#switch_theme(name)
 endfunction
 
 function! airline#switch_matching_theme()
+  if exists('g:colors_name')
     let v:errmsg = ''
     silent! let palette = g:airline#themes#{g:colors_name}#palette
     if empty(v:errmsg)
@@ -34,6 +35,7 @@ function! airline#switch_matching_theme()
         endif
       endfor
     endif
+  endif
   return 0
 endfunction