Audit improper usage of scheduler exposed by 5c713fdf.

channels/chan_iax2.c:
* Initialize struct chan_iax2_pvt scheduler ids earlier because of
iax2_destroy_helper().

channels/chan_sip.c:
channels/sip/config_parser.c:
* Fix initialization of scheduler id struct members.  Some off nominal
paths had 0 as a scheduler id to be destroyed when it was never started.

chan_skinny.c:
* Fix some scheduler id comparisons that excluded the valid 0 id.

channel.c:
* Fix channel initialization of the video stream scheduler id.

pbx_dundi.c:
* Fix channel initialization of the packet retransmission scheduler id.

ASTERISK-25476

Change-Id: I07a3449f728f671d326a22fcbd071f150ba2e8c8