Initialize variables before calling parse_uri

If parse_uri was called with an empty URI, some pointers would be
modified and an invalid read could result. This patch avoids calling
parse_uri with an empty contact uri when parsing REGISTER requests. 

AST-2011-012

(closes issue ASTERISK-18668)
........

Merged revisions 341189 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 341190 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@341191 65c4cc65-6c06-0410-ace0-fbb531ad65f3