diag: Ensure dci entry is valid before sending the packet

Possibility of a race condition which can free the dci entry
causing use after free case is prevented by adding the check
for entry's validity.

Change-Id: Ib436ffd16c266636d99885d6091eb1a6887737c7
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>