msm: ADSPRPC: Handle fastRPC context properly

Added NULL pointer check for context before
sending ack for glink message.

Change-Id: I37e0e44d06f1d0522f40c596df6357e0e5d135e4
Acked-by: Krishnaiah Tadakamalla <ktadakam@qti.qualcomm.com>
Signed-off-by: Jeya R <jeyr@codeaurora.org>