fixes problem with duplicate TXREQ packets

fixes problem with duplicate TXREQ packets

When Asterisk receives an IAX2 TXREQ packet, try_transfer()
will call store_by_transfercallno() to link the chan_iax2_pvt
struct into iax_transfercallno_pvts. If a duplicate TXREQ
packet is received for the same call, the pvt struct will be
linked into iax_transfercallno_pvts multiple times.  This patch
fixes this.  Thanks rain for debugging this and providing a patch!

(closes issue #16904)
Reported by: rain
Patches:
      iax2-double-txreq-fix.diff uploaded by rain (license 327)
Tested by: rain, dvossel

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@250394 65c4cc65-6c06-0410-ace0-fbb531ad65f3