res_fax: Fix FAXOPT(faxdetect) timeout option.

The fax detection timeout option did not work because basically the wrong
variable was checked in fax_detect_framehook().  As a result, the timer
would timeout immediately and disable fax detection.

* Fixed ignoring negative timeout values.  We'd complain and then go right
on using the negative value.

* Fixed destroy_faxdetect() in the off-nominal case of an incomplete
object creation.

* Added more range checking to FAXOPT(gateway) timeout parameter.

ASTERISK-26214 #close
Reported by: Richard Mudgett

Change-Id: Idc5e698dfe33572de9840bc68cd9fc043cbad976