msm: ADSPRPC: Fix buffer overflow for session

Fix to check the session count before accessing the
session buffer.

Change-Id: Iff98b6504c331d65f46c7934e09720f4522ea225
Acked-by: Bharath Kumar <bkumar@qti.qualcomm.com>
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>