app_confbridge: Fix reference to cfg in menu_template_handler

app_confbridge:  Fix reference to cfg in menu_template_handler

menu_template_handler wasn't properly accounting for the fact that
it might be called both during a load/reload (which isn't really
valid but not prevented) and by a dialplan function.  In both cases
it was attempting to use the "pending" config which wasn't valid in
the latter case.  aco_process_config is also partly to blame because
it wasn't properly cleaning "pending" up when a reload was done and
no changes were made.  Both of these contributed to a crash if
CONFBRIDGE(menu,template) was called in a dialplan after a reload.

* aco_process_config now sets info->internal->pending to NULL
  after it unrefs it although this isn't strictly necessary in the
  context of this fix.
* menu_template_handler now uses the "current" config and silently
  ignores any attempt to be called as a result of someone uses the
  "template" parameter in the conf file.

Luckily there's no other place in the codebase where
aco_pending_config is used outside of aco_process_config.

ASTERISK-25506 #close
Reported-by: Frederic LE FOLL
Change-Id: Ib349a17d3d088f092480b19addd7122fcaac21a7