Soluciona la aceptación de reINVITES antes de recibir ACK en vez de mandar un 491.
/* We have received a reINVITE on an incoming call to which we have sent a 200 OK but not yet received
* an ACK. According to RFC 5407, Section 3.1.4, the proper way to handle this race condition is to accept
* the reINVITE since we have established a dialog.
*/
/* Note that this will both clear the pendinginvite flag and cancel the
* retransmission of the 200 OK. Basically, we're accepting this reINVITE as both an ACK
* and a reINVITE in one request.
*/
channels/chan_sip.c