On Mac OS X or Windows, sending a SIGHUP to the server or a
asynchronous flush (triggered by flush_time), would cause the
server to crash.

The problem was that a hook used to detach client API handles
wasn't prepared to handle cases where the thread does not have
a associated session.

The solution is to verify whether the thread has a associated
session before trying to detach a handle.

mysql-test/r/federated_debug.result:
  Add test case result for Bug#47525
mysql-test/t/federated_debug-master.opt:
  Debug point.
mysql-test/t/federated_debug.test:
  Add test case for Bug#47525
sql/slave.cc:
  Check whether a the thread has a associated session.
sql/sql_parse.cc:
  Add debug code to simulate a reload without thread session.