chan_iax2: Fix saving the wrong expiry time in astdb.

When a new IAX2 client registers, the astdb database is updated with the
value of minregexpire defined in iax.conf instead of using the expiry time
that is provided by the client.  The provided expiry time of the client is
updated after inserting the astdb entry.  As a consequence, restarting or
reloading asterisk creates clients whose registration may expire before
they reregister.  The clients are therefore unavailable after minregexpire
seconds until they reregister.

* Move updating of the expiry time to before inserting into the astdb.

(closes issue ASTERISK-22504)
Reported by: Stefan Wachtler
Patches:
      chan_iax2.c.patch (license #6533) patch uploaded by Stefan Wachtler


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@399158 f38db490-d61c-443f-a65b-d21fe96a405b