Assertion Failure When Closing Database

Original [issue 136](https://code.google.com/p/leveldb/issues/detail?id=136) created by stephen.p.morgan on 2013-01-03T19:33:17.000Z:

What steps will reproduce the problem?
1. open array of dbs, "db[n]"
2. close db using "delete db[n]"

What is the expected output? What do you see instead?
Expect db to close. Instead, see assertion failure:

```
main: db/version_set.cc:715: leveldb::VersionSet::~VersionSet(): Assertion 'dummy_versions_.next_ == &dummy_versions_' failed.
```

What version of the product are you using? On what operating system?
1.6.0 on Ubuntu Linux kernel 3.2.1, x86_64

Please provide any additional information below.