chan_sip.c: Fix deadlock potential in fax redirection.

The sip_read() has the potential to deadlock if an incoming fax happens
during the Playback or similar application.

* Fixed the potential deadlock by not calling ast_async_goto() with the
channel lock held.

* Made always eat the fax detection frame whether there is a fax extension
or not.

ASTERISK-26216
Reported by: Richard Mudgett

Change-Id: I6d3f5cccd4b77c3aa6ffc1a54c0f6bde61c9278e