msm: ADSPRPC: Validate context pointer

Validate context pointer received from remote processor before
notifying users.

Change-Id: I9107f26031a963ed5266103d8312894762d25214
Acked-by: Viswanatham Paduchuri <vpaduchu@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>