dmaengine: at_xdmac: fix potential Oops in at_xdmac_prep_interleaved()

There are two place if the at_xdmac_interleaved_queue_desc() fails which
could lead to a NULL dereference where "first" is NULL and we call
list_add_tail(&first->desc_node, ...). In the first caller, the return
is not checked so add a check for that. In the next caller, the return
is checked but if it fails on the first iteration through the loop then
it will lead to a NULL pointer dereference.

Fixes: 4e5385784e69 ("dmaengine: at_xdmac: handle numf > 1")
Fixes: 62b5cb757f1d ("dmaengine: at_xdmac: fix memory leak in interleaved mode")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Tudor Ambarus <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/at_xdmac.c

···
 							NULL,
 							src_addr, dst_addr,
 							xt, xt->sgl);
+		if (!first)
+			return NULL;
 
 		/* Length of the block is (BLEN+1) microblocks. */
 		for (i = 0; i < xt->numf - 1; i++)
···
 							       src_addr, dst_addr,
 							       xt, chunk);
 			if (!desc) {
-				list_splice_tail_init(&first->descs_list,
-						      &atchan->free_descs_list);
+				if (first)
+					list_splice_tail_init(&first->descs_list,
+							      &atchan->free_descs_list);
 				return NULL;
 			}