bridging: Ensure locking during snapshot creation

While the vast majority of bridge snapshot creation is locked properly,
there are currently some instances that are not. This adds the missing
locking to ensure bridge state is not malleable during snapshot
creation.

(closes issue ASTERISK-22904)
Review: https://reviewboard.asterisk.org/r/3415/
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@412193 f38db490-d61c-443f-a65b-d21fe96a405b