[mucurses] Avoid potential division by zero

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/mucurses/slk.c

 		slks->spaces[0] = 3; slks->spaces[1] = 7;
 		break;
 	default:
-		nblocks = 0; nmaj = 0; nmin = 0;
-		break;
+		return ERR;
 	}
 
 	// determine maximum label length and major space size