[minor] set pkt = NULL inside loop in janus_ice_send_thread()

since pkt is now not always set at the beginning of the loop

(although it does seem to be cleared before every continue, so it was probably ok)

related to changes in https://github.com/meetecho/janus-gateway/commit/07e092b4eca86b759f05d2a15c7cfd73832fef79