Stasis: Only log errors for non-declined types

When message type creation is declined via stasis.conf, certain
operations log errors assuming that the declined type is being used
before initialization or after destruction. These error messages get
quite spammy for oft used message types and should not be logged in the
first place since the message type is validly NULL.

Reported by: Matt DiMeo


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424769 f38db490-d61c-443f-a65b-d21fe96a405b