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 #2710)

src/screen.c

@@ -8127,7 +8127,7 @@ screen_start_highlight(int attr)
 			term_bg_color(aep->ae_u.cterm.bg_color - 1);
 		}
 
-		if (t_colors <= 1)
+		if (!IS_CTERM)
 		{
 		    if (aep->ae_u.term.start != NULL)
 			out_str(aep->ae_u.term.start);

src/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

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1600,
 /**/
     1599,
 /**/