Fix GCC UBSan (SANITIZE=1 build) runtime error

left shift of 255 by 24 places cannot be represented in type 'int'

src/conio/bitmap_con.c

@@ -1931,7 +1931,7 @@ int bitmap_drv_init(void (*drawrect_cb) (struct rectlist *data)
 	vstat.flags = VIDMODES_FLAG_PALETTE_VMEM;
 	pthread_mutex_lock(&screenlock);
 	for (i = 0; i < sizeof(dac_default)/sizeof(struct dac_colors); i++) {
-		palette[i] = (0xff << 24) | (dac_default[i].red << 16) | (dac_default[i].green << 8) | dac_default[i].blue;
+		palette[i] = (0xffU << 24) | (dac_default[i].red << 16) | (dac_default[i].green << 8) | dac_default[i].blue;
 	}
 	pthread_mutex_unlock(&screenlock);
 	pthread_mutex_unlock(&vstatlock);