sip: fix pkt receive

- Sending profile highlighted a flaw in the processing of packet
  * The receive packet function from pjsip can't handle a packet
    larger than PJSIP_MAX_PKT_LEN (4000) but we were not enforcing
    that correctly
  * The received callback was appending packet to the last one instead
    of appending it to the received packet vector

Change-Id: I603cf19cb2951708edd5cca44d71b77bb83114d6
Tuleap: #538