Check for existence of key in context dict before using it
Check for existence of key in context dict before using it

fixes #1251

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