Bridging: Don't lock NULL bridges

When bridge locking was added for bridge snapshot creation, some
locations where bridge locking was added were not guaranteed to
actually have a bridge and locking NULL AO2 objects tends to cause
segfaults. This ensures that NULL bridges aren't locked.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@413073 f38db490-d61c-443f-a65b-d21fe96a405b