Use locking when accessing the registrations list. This list is not actually

Use locking when accessing the registrations list.  This list is not actually
used very often, so the likelihood of there being a problem is pretty small,
but still possible.  For example, if the CLI command to list the registrations
was called at the same time that a reload was occurring and the registrations
list was getting destroyed and rebuilt, a crash could occur.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@48361 65c4cc65-6c06-0410-ace0-fbb531ad65f3