Check that default value exists, before accessing it
Check that default value exists, before accessing it

closes #1146

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