spi: check ctrl->cur_msg validity before dereferencing

In case an SPI transfer fails the mesg has been identified on SAMA5D2
ICP as being NULL. Thus check its validity to avoid a crash.

Fixes: 8caab75 ("spi: Generalize SPI "master" to "controller"")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>

Author: claudiubeznea

drivers/spi/spi.c

@@ -2001,6 +2001,8 @@ void spi_finalize_current_message(struct spi_controller *ctlr)
 	int ret;
 
 	mesg = ctlr->cur_msg;
+	if (!mesg)
+		return;
 
 	if (!ctlr->ptp_sts_supported && !ctlr->transfer_one) {
 		list_for_each_entry(xfer, &mesg->transfers, transfer_list) {