It can cause panic while running test data with some special

I found the test code below can cause the panic of DB, is any wrong with the test code?
```
type ritem struct {
Key string
Value []byte
}
base := []byte{1, 0, 1, 0}
key := append([]byte{}, base...)
minKey := []byte{1, 0, 1}

schema := &memdb.DBSchema{
Tables: map[string]*memdb.TableSchema{
"0": &memdb.TableSchema{
Name: "0",
Indexes: map[string]*memdb.IndexSchema{
"id": &memdb.IndexSchema{
Name: "id",
Unique: true,
Indexer: &memdb.StringFieldIndex{Field: "Key"},
},
},
},
},
}
mdb, err := memdb.NewMemDB(schema)
assert.Nil(t, err)

txn := mdb.Txn(true)
txn.Insert("0", &ritem{Key: string(key), Value: []byte("1")})
txn.Insert("0", &ritem{Key: string(append(key, byte(1))), Value: []byte("1")})
txn.Commit()

txn = mdb.Txn(false)
defer txn.Abort()
resIter, err := txn.LowerBound("0", "id", string(minKey)) // panic happened here
assert.Nil(t, err)
```

Reproduce the panic with the provided Go schema, byte keys, inserts, and read transaction, focusing on txn.LowerBound("0", "id", string(minKey)). Trace how LowerBound handles the special key prefix and determine the expected behavior; done means the example no longer panics and the behavior is covered by a regression test.

Written by the indexing model from the issue text.