patch 8.2.0891: clang warns for invalid conversion

Problem:    Clang warns for invalid conversion.
Solution:   Use zero instead of INVALCOLOR.
diff --git a/src/highlight.c b/src/highlight.c
index 852fffb..9ce6102 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -2235,7 +2235,7 @@
     at_en.ae_attr = attr;
     at_en.ae_u.cterm.fg_color = fg;
     at_en.ae_u.cterm.bg_color = bg;
-    at_en.ae_u.cterm.ul_color = INVALCOLOR;
+    at_en.ae_u.cterm.ul_color = 0;
     return get_attr_entry(&cterm_attr_table, &at_en);
 }
 #endif