chan_sip: Use AST_AF* defined constant when calling ast_get_ip

While the structure passed to ast_get_ip should be set memset to 0, thus
initializing the ss_family member to 0, explicitly setting it to AST_AF_UNSPEC
is more portable.
........

Merged revisions 402507 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402508 65c4cc65-6c06-0410-ace0-fbb531ad65f3