Skip stack and empty maps when reading load base.

When doing a local unwind, the stack maps and empty maps could be
changing. Since we try and read data from the load base from
every map, it's possible these two types of maps could disappear
between the time we read the maps and when we try and read the load
base from the map. Therefore, skip these transient maps.

Bug: 21949571
(cherry picked from commit 408bec05)

Change-Id: Ie789eadcf3a8a4354d46c7bb8d81b1af8dd7800c