Fixed startup race condition which caused occasional stasis_mwi_state_type assertions.

The caching topic (which refers to the message type) was created before the
message type. If the initial subscription message gets processed before
the type can be initialized, the assertion about using an uninitialized type
fires.


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389519 f38db490-d61c-443f-a65b-d21fe96a405b