dmaengine: at_xdmac: avoid null_prt_deref in at_xdmac_prep_dma_memset

In the Linux kernel, the following vulnerability has been resolved:

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.