Add MapAnonymous handling for null error_str

We use MapAnonymous with null error_str for app image loading when
we want to fail quickly. Also avoid doing CheckNonOverlapping in
CheckMapRequest if error_msg is null. The result from
CheckNonOverlapping is unused and CheckNonOverlapping is slow since
it creates a backtrace map.

Bug: 22858531
Bug: 26746779
Change-Id: I8605ec0b9d9ae554a4b74f2606fa7dd81ade9021