Fix segmentation fault

This adds some more accurate checks to avoid a segmentation fault that
occurred when accessing a nonexistent item.

Fixes GitHub issue #7.

Reported-by: Bromind <martin.vassor@hotmail.fr>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/args.c

@@ -309,7 +309,7 @@ app_arg(int add_line, struct date *day, long date, const char *fmt_apt,
 
 	day_store_items(date, regex, 0);
 
-	int n = day_item_count();
+	int n = day_item_count(0);
 
 	if (n > 0) {
 		if (add_line)