only release the lock if it is defined, to avoid deleting another thread/procs lock
only release the lock if it is defined, to avoid deleting another thread/procs lock

Percona-Lab/mongodb_consistent_backup,timvaillancourt/mongodb_consistent_backup,Percona-Lab/mongodb_consistent_backup,Percona-Lab/mongodb_consistent_backup,timvaillancourt/mongodb_consistent_backup,timvaillancourt/mongodb_consistent_backup