msm: Add null-pointer checks for domains

Add check for null-pointer before passing domain pointer
into map and unmap api to avoid null-pointer deferences.

CRs-fixed: 493503
Change-Id: I185deb9f2878ef62601eb4ca1d2b71f892ca0185
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>