prima: memset the command buffer in GetCommandBuffer routine

Currently command buffer returned by GetCommandBuffer routine is
not memset to zero. If all the fields are not populated, there
could be some stale entries which could result in crash.
This change set fixes the issue by setting command buffer to zero
before returning it to the caller.

Change-Id: I9b9120485ae2eef9d4f4cf04889998c3e2af1d9d
CRs-Fixed: 791618