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: http://svn.asterisk.org/svn/asterisk/trunk@391902 f38db490-d61c-443f-a65b-d21fe96a405b