Ensure that the number of added contacts never goes below 0.

This can happen when a REGISTER request is removing a contact.

(closes issue ASTERISK-21911)
Reported by: mdavenport


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391902 65c4cc65-6c06-0410-ace0-fbb531ad65f3