channeldb: copy value from boltdb's Get instead of using it directly

This can cause an intermittent panic otherwise if bbolt remaps itself
via munmap and mmap. From bbolt's documentation:

* Byte slices returned from Bolt are only valid during a transaction.
Once the transaction has been committed or rolled back then the memory
they point to can be reused by a new page or can be unmapped from
virtual memory and you'll see an unexpected fault address panic when
accessing it.

channeldb/db.go

@@ -1254,11 +1254,13 @@ func (c *ChannelStateDB) GetChannelOpeningState(outPoint []byte) ([]byte, error)
 			return ErrChannelNotFound
 		}
 
-		serializedState = bucket.Get(outPoint)
+		stateBytes := bucket.Get(outPoint)
-		if serializedState == nil {
+		if stateBytes == nil {
 			return ErrChannelNotFound
 		}
 
+		serializedState = append(serializedState, stateBytes...)
+
 		return nil
 	}, func() {
 		serializedState = nil