Fix problem adding too many frames.

When adding a frame with a dex pc, two frames will be added total. However,
if there is only enough room for a single frame, two get added any way.

Only add a single frame in this case, and add a unit test for this case.

Test: Passes unit tests.
Change-Id: If320584b126967a042c623d8fdf3f51dbc1c2251