stored function invoked from different connections".

Invocation of trigger which was using stored function from different
connections caused server crashes (for non-debug server this happened
in highly concurrent environment, but debug server failed on assertion
in relatively simple scenario).

Item_func_sp was not safe to use in triggers (in other words for
re-execution from different threads) as artificial TABLE object
pointed by Item_func_sp::dummy_table referenced incorrect THD
object. To fix the problem we force re-initialization of this
object for each re-execution of statement.


mysql-test/r/trigger.result:
  Added test for bug#23651 "Server crashes when trigger which uses
  stored function invoked from different connections".
mysql-test/t/trigger.test:
  Added test for bug#23651 "Server crashes when trigger which uses
  stored function invoked from different connections".
sql/item_func.cc:
  To make Item_func_sp safe for usage in triggers (in other words safe
  for re-execution in different threads) we need to ensure that artificial
  TABLE object pointed by Item_func_sp::dummy_table references correct
  THD object. To achieve this we simply force its re-initialization for
  each re-execution of statement.