fix crash on invalid tags file

If the tag entry points to the line number zero, less crashes because
internally it uses that line number to imply that the tag is associated
with a pattern.

issue reported by Henry Ford (henryfordkjv at gmail), thanks!

ok kirill@

usr.bin/less/tags.c

@@ -260,6 +260,10 @@ findctag(char *tag)
 		 */
 		tagendline = 0;
 		taglinenum = getnum(&p, 0, &err);
+		if (!err && taglinenum <= 0) {
+			/* line numbers must be positive */
+			continue;
+		}
 		if (err) {
 			/*
 			 * No, it must be a pattern.