Bluetooth: Validate session before rfcomm_session_del

In rfcomm_session_put API the session count is decremented
and when it is equal to zero rfcomm_session_del is called
where session is removed from sessions list and freed. The
current change is verify the existance of session in list
before acting session. The avoids the possibility of action
on a deleted session, which causes crash.

CRs-Fixed: 383000
Change-Id: Ia55607b08ee388465494f08bbe1627102d281f8a
Signed-off-by: Srinivas Krovvidi <skrovvid@codeaurora.org>