patch 9.1.0139: MS-Windows: ligatures map cleared on startup

patch 9.1.0139: MS-Windows: ligatures map cleared on startup

Problem:  MS-Windows: ligatures map cleared on startup
Solution: Don't clear the ligatures_map, if the guiligatures
          option has already been set (Erik S. V. Jansson)

If guiligatures is set from a .vimrc it's possible
that it will be cleared on start-up (e.g. in Win32
that's what happens).

So don't clear the ligatures map if gui_set_ligatures()
has already been called (e.g. after setting 'ligatures'
from .vimrc)

closes: #14094

Signed-off-by: Erik S. V. Jansson <caffeineviking@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>