diag: Add protection while accessing diag client map

There is a possibility of use-after-free while accessing
diag client map table since list can be reallocated due to
exceeding max client limit. The patch adds protection
while accessing client map list.

Change-Id: Ibc33d9d90b7e22394c82265b0fa80a89cc2a3122
Signed-off-by: Hardik Arya <harya@codeaurora.org>