getline: add check for len > 0

This condition cannot happen here though.

dmenu.c

@@ -562,7 +562,7 @@ readstdin(void)
 			if (!(items = realloc(items, itemsiz * sizeof(*items))))
 				die("cannot realloc %zu bytes:", itemsiz * sizeof(*items));
 		}
-		if (line[len - 1] == '\n')
+		if (len > 0 && line[len - 1] == '\n')
 			line[len - 1] = '\0';
 		if (!(items[i].text = strdup(line)))
 			die("strdup:");