Audit improper usage of scheduler exposed by 5c713fdf. (v13 additions)

chan_sip.c:
* Initialize mwi subscription scheduler ids earlier because of ASTOBJ to
ao2 conversion.

* Initialize register scheduler ids earlier because of ASTOBJ to ao2
conversion.

chan_skinny.c:
* Fix more scheduler usage for the valid 0 id value.

ASTERISK-25476

Change-Id: If9f0e5d99638b2f9d102d1ebc9c5a14b2d706e95