dmaengine: ptdma: check for null desc before calling pt_cmd_callback

In the Linux kernel, the following vulnerability has been resolved: dmaengine: ptdma: check for null desc before calling pt_cmd_callback Resolves a panic that can occur on AMD systems, typically during host shutdown, after the PTDMA driver had been exercised. The issue was the pt_issue_pending() function is mistakenly assuming that there will be at least one descriptor in the Submitted queue when the function is called. However, it is possible that both the Submitted and Issued queues could be empty, which could result in pt_cmd_callback() being mistakenly called with a NULL pointer. Ref: Bugzilla Bug 216856.