Need to sanity test mailsystem directory in update function

When I tried to upgrade my mailsystem to 3.x (from 2.34), the upgrade ended up deleting hundreds of core and module files across my entire install.

This is because I have s3fs installed, and the call to file_build_uri('mailsystem') in the upgrade function ends up returning "s3://mailsystem" (a clear error), but then drupal_realpath converts that to an empty string, leaving class_dir as "/". Thus the sweep through ended up deleting every file with a class in my entire install. Gah!

Thankfully this was just a dev install, but what a nightmare. A simple check to make sure that drupal_realpath(file_build_uri('mailsystem')) doesn't return an empty string would remove the possibility of this disaster for future users.