Region: Speculative fix for ubsan abort

Adds a special case to Region::end() to handle mStorage being empty.

By design, this should never happen, but something seems to be causing
it to happen, so instead of allowing the ubsan abort to take down
SurfaceFlinger, we work around the issue by special-casing size() == 0.

Bug: 77643177
Test: By design this shouldn't happen, so it's not yet clear how to
      write a test to exercise it

Change-Id: Ifaae2eb8e89460319e599205cd64f00d120279b8