chan_sip: Unref outbound proxy structure on dialog/pvt destruction.

Make sure outbound proxy refs are always unreffed on dialog destruction.

Review: https://reviewboard.asterisk.org/r/4016/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@423800 f38db490-d61c-443f-a65b-d21fe96a405b