UDPTL: Backport some fixes from v12 that should be in v11.

Backported the following as applied to udptl.c:
* -r398020 Fixup udpdl defaults if config file not present.
* -r398533 Fixup improper use of ao2_global_obj_replace().


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@399442 f38db490-d61c-443f-a65b-d21fe96a405b