spi: dw-mid: avoid potential NULL dereference

commit c9dafb27 upstream.

When DMA descriptor allocation fails we should not try to assign any fields in
the bad descriptor. The patch adds the necessary checks for that.

Fixes: 7063c0d9 (spi/dw_spi: add DMA support)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/spi/spi-dw-mid.c

@@ -139,6 +139,9 @@ static struct dma_async_tx_descriptor *dw_spi_dma_prepare_tx(struct dw_spi *dws)
 				1,
 				DMA_MEM_TO_DEV,
 				DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+	if (!txdesc)
+		return NULL;
+
 	txdesc->callback = dw_spi_dma_tx_done;
 	txdesc->callback_param = dws;
 
@@ -184,6 +187,9 @@ static struct dma_async_tx_descriptor *dw_spi_dma_prepare_rx(struct dw_spi *dws)
 				1,
 				DMA_DEV_TO_MEM,
 				DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+	if (!rxdesc)
+		return NULL;
+
 	rxdesc->callback = dw_spi_dma_rx_done;
 	rxdesc->callback_param = dws;