diag: Add validity check for process descriptor

This fix checks the validity of memory device session's
process descriptor before issuing a signal to it while
subsystem restart is performed. This fix avoids accessing
of cleaned-up process descriptor's fields.

CRs-Fixed: 2034816
Change-Id: I2725d3348fbeb8290c887cccea4bca6d41c5b184
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>