msm: adsprpc: Validate buffer pointer before flush

Check for valid map->buf before calling flush on these buffers.

Change-Id: Ifab138d3523f4f56df97724ef418e3cc40bb925c
Acked-by: Jeya R <jeyr@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>