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.