nil pointer dereference in mongoreplay when error on new playback file creation

If there is an error when creating a new playback writer, there is a nil pointer dereference on line 172:
https://github.com/mongodb/mongo-tools/blob/fd1aa55895cc2ca774bea5ac737177d80b4c7ff3/mongoreplay/record.go#L172

the defer should be after the error check because arguments to defer are evaluated eagerly.