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)
diff --git a/src/version.c b/src/version.c
index eabf6fb..23b0168 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1600,
+/**/
     1599,
 /**/
     1598,