Scenario:
Local fax -> Asterisk w/ firewall -> Provider -> Remote fax

* Local fax starts rtp call to remote fax
* Remote fax starts t38 call back to local fax.
* Local fax sends t38 no-signal to Asterisk before sending an OK.
* udptl processes the frame and increments the expected sequence number.
* chan_sip drops the frame because the call isn't up so nothing goes out
  the external interface to open the port for incoming packets.
* Local fax sends OK and Asterisk sends OK to the remote fax.
* Remote fax sends t38 packets which are dropped by the firewall.
* Local fax re-sends t38 no-signal with the same sequence number.
* udptl drops the frame because it thinks it's a dup.
* Still no outgoing packets to open the firewall.
* t38 negotiation fails.

The patch drops frames t38 received before udptl sequence processing
when the call hasn't been answered yet.  The second no-signal frame
is then seen as new and is relayed out the external interface which
opens the port and allows negotiation to continue.

ASTERISK-26034 #close

Change-Id: I11744b39748bd2ecbbe8ea84cdb4f3c5943c5af9