context.

Routine arguments were evaluated in the security context of the routine
itself, not in the caller's context.

The bug is fixed the following way:

  - Item_func_sp::find_and_check_access() has been split into two
    functions: Item_func_sp::find_and_check_access() itself only
    finds the function and check that the caller have EXECUTE privilege
    on it.  New function set_routine_security_ctx() changes security
    context for SUID routines and checks that definer have EXECUTE
    privilege too.

  - new function sp_head::execute_trigger() is called from
    Table_triggers_list::process_triggers() instead of
    sp_head::execute_function(), and is effectively just as the
    sp_head::execute_function() is, with all non-trigger related code
    removed, and added trigger-specific security context switch.

  - call to Item_func_sp::find_and_check_access() stays outside
    of sp_head::execute_function(), and there is a code in
    sql_parse.cc before the call to sp_head::execute_procedure() that
    checks that the caller have EXECUTE privilege, but both
    sp_head::execute_function() and sp_head::execute_procedure() call
    set_routine_security_ctx() after evaluating their parameters,
    and restore the context after the body is executed.