pjsip_distributor.c: Ignore messages until fully booted.

We should not be processing any incoming messages until we are fully
booted.  We may not have dialplan or other needed configuration loaded
yet.

ASTERISK-26089 #close
Reported by: Scott Griepentrog

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: I584aefb4f34b885a8927e1f13a2c64babd606264