whisper: fix issue in topic list copy (#16381)

- Fixes #16271. What was appeneded was a pointer to
an object that changes during the iteration.
- The topic is allocated as a 4-byte array, fill partial topics
with 0s. Partial topics are currently disabled, but would
crash as they rely on the presence of byte number 3.

whisper/whisperv6/api.go

@@ -558,9 +558,10 @@ func (api *PublicWhisperAPI) NewMessageFilter(req Criteria) (string, error) {
 	}
 
 	if len(req.Topics) > 0 {
-		topics = make([][]byte, 0, len(req.Topics))
-		for _, topic := range req.Topics {
-			topics = append(topics, topic[:])
+		topics = make([][]byte, len(req.Topics))
+		for i, topic := range req.Topics {
+			topics[i] = make([]byte, TopicLength)
+			copy(topics[i], topic[:])
 		}
 	}