dmaengine: at_xdmac: avoid null_prt_deref in at_xdmac_prep_dma_memset

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 <chenridong@huawei.com>
Link: https://lore.kernel.org/r/20241029082845.1185380-1-chenridong@huaweicloud.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>