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.


sql/slave.cc:
  Changing signature of terminate_slave_thread() to accept a skip_lock
  parameter instead of two mutexes. This mimics the signature of the
  terminate_slave_threads() function. Code is also changed as a result
  of this.
  
  Removing unguarded check of slave_running field in the master info and
  relay log info structure since that could cause premature exit of
  terminate_slave_threads().
  
  The thread variable for each of the slave threads can change before
  acquiring the run_lock mutex inside terminate_slave_thread(). Hence
  an assertion was removed that read the variable without guarding it
  with run_lock.
  
  Code that checked *slave_running status inside terminate_slave_thread()
  was reorganized slightly.
sql/slave.h:
  Moving terminate_slave_thread() to use internal linkage.