Removing unguarded read of slave_running field from inside
terminate_slave_threads(). This could cause premature exit in the event
that the slave thread already were shutting down, but isn't finished yet.

The fields slave_running, io_thd, and sql_thread are guarded by an
associated run_lock. A read of these fields were not guarded inside
terminate_slave_threads(), which caused an assertion to fire. The
assertion was removed, and the code reorganized slightly.