vim-patch:8.0.1600: crash when setting t_Co to zero when 'termguicolors' is set

Problem:    Crash when setting t_Co to zero when 'termguicolors' is set.
Solution:   Use IS_CTERM instead of checking the number of colors.
            (closes vim/vim#2710)
f708ac592f

src/nvim/testdir/test_highlight.vim

@@ -514,3 +514,17 @@ func Test_highlight_eol_on_diff()
   bwipe!
   diffoff
 endfunc
+
+func Test_termguicolors()
+  if !exists('+termguicolors')
+    return
+  endif
+
+  " Basic test that setting 'termguicolors' works with one color.
+  set termguicolors
+  redraw
+  set t_Co=1
+  redraw
+  set t_Co=0
+  redraw
+endfunc