The problem is a race between a session closing its vio
(i.e. after a COM_QUIT) at the same time it is being killed by
another thread. This could trigger a assertion in vio_close()
as the two threads could end up closing the same vio, at the
same time. This could happen due to the implementation of
SIGNAL_WITH_VIO_CLOSE, which closes the vio of the thread
being killed.

The solution is to serialize the close of the Vio under
LOCK_thd_data, which protects THD data.

No regression test is added as this is essentially a debug
issue and the test case would be quite convoluted as we would
need to synchronize a session that is being killed -- which
is a bit difficult since debug sync points code does not
synchronize killed sessions.

sql/mysqld.cc:
  Drop lock parameter from close_connection, its not necessary
  any more. The newly introduced THD::disconnect method will take
  care of locking.
sql/mysqld.h:
  Change prototype, add a default parameter for the error code.
sql/sql_class.cc:
  In case SIGNAL_WITH_VIO_CLOSE is defined, the active vio is
  closed and cleared. Subsequent calls will only close the vio
  owned by the session.