Fix 'conceal' by matchadd() when :syn off

To be precise, when there are no text which matches syntax
definition in current buffer.

src/screen.c

@@ -4910,7 +4910,18 @@ win_line(
 				    && vim_strchr(wp->w_p_cocu, 'v') == NULL))
 	    {
 		char_attr = conceal_attr;
-		if (prev_syntax_id != syntax_seqnr
+		/* PROBLEM: When trying to show cchar by matchadd(),
+		 * prev_syntax_id and syntax_seqnr might be 0 with :syntax off.
+		 * When trying to show cchar by :syntax match, it's no problem because
+		 * syntax_seqnr is not 0.
+		 * To support showing cchar by matchadd(), change syntax_seqnr
+		 * value to non-zero.
+		 * XXX: better solution?
+		 **/
+		if (has_match_conc && syntax_seqnr == 0)
+		    syntax_seqnr = -1;
+
+		if ((prev_syntax_id != syntax_seqnr)
 			&& (syn_get_sub_char() != NUL || match_conc
 							 || wp->w_p_cole == 1)
 			&& wp->w_p_cole != 3)