diag: dci: Add validity check for dci client's process descriptor

This fix checks the validity of dci client's process descriptor
before issuing a signal to it when subsystem restart is performed.
This fix avoids accessing cleaned-up process descriptor's fields.

CRs-Fixed: 2047235
Change-Id: Ic26977dc22c68f0a7007dd963c9273bba2a5dbfe
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>