scsi: ufs: core: Add NULL check in ufshcd_mcq_compl_pending_transfer()

In the Linux kernel, the following vulnerability has been resolved: scsi: ufs: core: Add NULL check in ufshcd_mcq_compl_pending_transfer() Add a NULL check for the returned hwq pointer by ufshcd_mcq_req_to_hwq(). This is similar to the fix in commit 74736103fb41 ("scsi: ufs: core: Fix ufshcd_abort_one racing issue").