gui_running may not be set for neovim

[matthew-brett]

See discussion of "gui_running" and neovim at
neovim/neovim#3646 and related fix at
junegunn/goyo.vim@8faad54

Without this fix, running Neovim in GUI mode via neovim-dot-app, I get
messages of form:

Color name or number not recognized: ctermfg=#b1d631

[alerque]

I've been running Neovim in both GUI and terminal true color environments for years along with this plugin and haven't encountered this. Under what circumstances is this a problem? In other words how can I replicate the scenario this effects?

[matthew-brett]

From the discussion linked, it looks like gui_running may or may not be available at startup, so I guess that means this error will happen only in some cases of a GUI app.

I'm running Neovim via https://github.com/rogual/neovim-dot-app with the mustang color scheme. But I get the same effect on one other colorscheme I tried.

I don't know Vim colors very well, but I thought that it was always true that a ctermfg= followed by a #RGB is going to give an error, so I guess this change is safe.