in trigger

Interchangeable calls to the mysql_change_user client function
and invocations of a trigger changing some user variable caused
a memory corruption and a crash.

The mysql_change_user API call forces TDH::cleanup() on a server
that frees user variable entries.
However it didn't reset Item_func_set_user_var::entry to NULL
because Item_func_set_user_var::cleanup() was not overloaded.
So, Item_func_set_user_var::entry held a pointer to freed memory,
that caused a crash.

The Item_func_set_user_var::cleanup method has been overloaded
to cleanup the Item_func_set_user_var::entry field.


mysql-test/r/user_var.result:
  Added test case for bug #42188.
mysql-test/t/user_var.test:
  Added test case for bug #42188.
sql/item_func.cc:
  Bug#42188: crash and/or memory corruption with user variables
             in trigger
  
  The Item_func_set_user_var::cleanup method has been overloaded
  to cleanup the Item_func_set_user_var::entry field.
sql/item_func.h:
  Bug#42188: crash and/or memory corruption with user variables
             in trigger
  
  The Item_func_set_user_var::cleanup method has been overloaded
  to cleanup the Item_func_set_user_var::entry field.