Fixes for BUG11761686 left a flaw that managed to slip away from testing.
Only effective filtering branch was actually tested with a regression test
added to rpl_filter_tables_not_exist.
The reason of the failure is destuction of too early mem-root-allocated memory 
at the end of the deferred User-var's do_apply_event().

Fixed with bypassing free_root() in the deferred execution branch.
Deallocation of created in do_apply_event() items is done by the base code
through THD::cleanup_after_query() -> free_items() that the parent Query
can't miss.



sql/log_event.cc:
  Do not call free_root() in case the deferred User-var event.
  Necessary methods to the User-var class are added, do_apply_event() refined.
sql/log_event.h:
  Necessary methods to avoid destoying mem-root-based memory at
  User-var applying are defined.