res_pjsip_t38.c: Fix crash by ignoring 1xx messages.

If we initiated a T.38 reINVITE, we would crash if we received any other
1xx response message except 100 if it were followed by a 200 response.

* Made ignore any 1xx response so we do not close out the T.38 negotiation
too early.  For good measure we'll now accept any 2xx response as
acceptance of the reINVITE T.38 offer.

ASTERISK-27944

Change-Id: I0ca88aae708d091db7335af73f41035a212adff4