pjsip: fixed compilation warning

Ignore-this: 5528f005cb08cb813d533cd301d894b3

darcs-hash:20140226115745-56b4d-b37d899c1d9676d1c026983ebf7f28f3a41cb2ae
Handle errors when sending hold/unhold requests

Ignore-this: b8c44298bbc64d8c9a4c514dc40caee1

darcs-hash:20140227134445-56b4d-a5e908d669daea3d2c6b6cecbd13241f786665f4