Update inmem_store.go
Update inmem_store.go

InmemStore implements the LogStore and StableStore interface.
However it's implementation of the StableStore interface Get method has a bug. 

If you look at the hashicorp/raft-boltdb[1] and hashicorp/raft-mdb[2] implementation of the same method; 
then if a get for a key returns a nil byte, that method should return an error. 

Since InmemStore is mostly used in tests, it's implementation should be aligned with raft-boltdb raft-mdb so as not hide any errors

ref:
1. https://github.com/hashicorp/raft-boltdb/blob/6e5ba93211eaf8d9a2ad7e41ffad8c6f160f9fe3/bolt_store.go#L241-L246
2. https://github.com/hashicorp/raft-mdb/blob/55f29473b9e604b3678b93a8433a6cf089e70f76/mdb_store.go#L328-L330
otoolep/raft,hashicorp/raft,hashicorp/raft