Handle null state on close in FileMaker

If getLine() is not called due to some other exception in AbstractRepositoryI,
then FileMaker.close() should not blindly try to close the lock.

To reproduce, make the "lib/scripts" directory read-only for the
omero user, and restart the server. Rather than a NPE, there should
be an error in the log stating:

```
013-03-21 16:24:40,880 ERROR [.services.blitz.repo.AbstractRepositoryI]
(2-thread-3) Failed during repository takeover
java.io.FileNotFoundException: ./lib/scripts/.omero/repository/71dfdc54-d5f3-4be7-86ef-6f19c9397e8c/repo_uui
d (No such file or directory)
    at java.io.RandomAccessFile.open(Native Method)
    at java.io.RandomAccessFile.<init>(Unknown Source)
    at ome.services.blitz.repo.FileMaker.init(FileMaker.java:82)
```

Currently debating how best to handle the read-only lib/scripts dir.

/cc @will-moore