database cursor not closed in certain scenarios
Bug reported on oC tracker, also valid for Nc:

---
### Actual behaviour

-database cursor may be left open
### Expected behaviour

-cursors even empty ones (moveToFirst returns false) should be closed

The database cursor used by the delete() method (Line 94) in FileContentProvider class may be left unclosed if cursor.moveToFirst() returns false:

https://github.com/owncloud/android/blob/master/src/com/owncloud/android/providers/FileContentProvider.java

```
if (c != null && c.moveToFirst()) {
                    remoteId = c.getString(c.getColumnIndex(ProviderTableMeta.FILE_REMOTE_ID));
                    //ThumbnailsCacheManager.removeFileFromCache(remoteId);
                    c.close();
                }
```

---

I'll open a PR right away.
