before trying to return hi attributes, check the group exists
before trying to return hi attributes, check the group exists

This prevents trying to access twice the highlighting groups
and should slightly speed up airline.

tschundler/dotfiles,unphased/vim-airline,tschundler/dotfiles,tschundler/dotfiles,rchicoli/vim-airline,bling/vim-airline,chrisbra/vim-airline,yzhou61/vim-airline,HarmtH/vim-airline