panic in db.Stats() during db.Close()

[StephenButtolph]

There appears to be a panic bug/race during db.Close() + db.Stat().

1. The first thing done in db.Stat() is make sure the db isn't closed: https://github.com/syndtr/goleveldb/blob/master/leveldb/db.go#L1075-L1078
2. If db.Close() is then called (after the aforementioned check), we have this trace:
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/db.go#L1218
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/session.go#L101
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/table.go#L496
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/cache/cache.go#L647
3. Then we can call GetStats() on the now closed cache resulting in this trace:
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/db.go#L1095
   • https://github.com/syndtr/goleveldb/blob/master/leveldb/cache/cache.go#L427 which panics because the head was set to nil in the prior close operation.

I think the core invariant break is documented here:

• https://github.com/syndtr/goleveldb/blob/master/leveldb/cache/cache.go#L635

MVP test which panics:

package leveldbtest

import (
	"testing"

	"github.com/syndtr/goleveldb/leveldb/cache"
)

func TestLRUCache_GetStatsAfterClose(t *testing.T) {
	c := cache.NewCache(cache.NewLRU(2))
	c.Close(false)
	_ = c.GetStats() // panics
}