diag: Validate memory device client's process descriptor

This fix checks for valid process descriptor of a
memory device client before exporting information
from diag driver to memory device client's read buffer
for reliable data transfer.

CRs-Fixed: 2016396
Change-Id: I45aeb8fc9e2f6a678d48bbfcbb77c501adbbfce0
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>