spi: imx: fix issue when tx_buf or rx_buf is NULL

In case either transfer->tx_buf or transfer->rx_buf is NULL,
manipulation of buffer in spi_imx_u32_swap_u[8|16]() will cause
NULL pointer dereference crash.

Add buffer check at very beginning of spi_imx_u32_swap_u[8|16](),
to avoid such crash.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -288,6 +288,9 @@ static void spi_imx_u32_swap_u8(struct spi_transfer *transfer, u32 *buf)
 {
 	int i;
 
+	if (!buf)
+		return;
+
 	for (i = 0; i < transfer->len / 4; i++)
 		*(buf + i) = cpu_to_be32(*(buf + i));
 }
@@ -296,6 +299,9 @@ static void spi_imx_u32_swap_u16(struct spi_transfer *transfer, u32 *buf)
 {
 	int i;
 
+	if (!buf)
+		return;
+
 	for (i = 0; i < transfer->len / 4; i++) {
 		u16 *temp = (u16 *)buf;