Fix for chan_skinny leaving RTP ports open

Skinny wasn't closing RTP sockets. This patch includes ast_rtp_instance_stop before 
ast_rtp_instance_destroy which fixes the problem. Also add destroy for VRTP (which 
I believe is unused, but exists).

Review: https://reviewboard.asterisk.org/r/2176/
........

Merged revisions 375660 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375663 65c4cc65-6c06-0410-ace0-fbb531ad65f3