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/version.c

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