dmaengine: at_xdmac: avoid null_prt_deref in at_xdmac_prep_dma_memset

The at_xdmac_memset_create_desc may return NULL, which will lead to a
null pointer dereference. For example, the len input is error, or the
atchan->free_descs_list is empty and memory is exhausted. Therefore, add
check to avoid this.

Fixes: b206d9a23ac7 ("dmaengine: xdmac: Add memset support")
Signed-off-by: Chen Ridong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/at_xdmac.c

···
 		return NULL;
 
 	desc = at_xdmac_memset_create_desc(chan, atchan, dest, len, value);
+	if (!desc)
+		return NULL;
 	list_add_tail(&desc->desc_node, &desc->descs_list);
 
 	desc->tx_dma_desc.cookie = -EBUSY;