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.