whisper/whisper2: fix Go 1.10 vet issues on type mismatches (#15783)

whisper/whisperv2/whisper.go

@@ -262,7 +262,7 @@ func (self *Whisper) add(envelope *Envelope) error {
 	// Insert the message into the tracked pool
 	hash := envelope.Hash()
 	if _, ok := self.messages[hash]; ok {
-		log.Trace(fmt.Sprintf("whisper envelope already cached: %x\n", envelope))
+		log.Trace(fmt.Sprintf("whisper envelope already cached: %x\n", hash))
 		return nil
 	}
 	self.messages[hash] = envelope
@@ -277,7 +277,7 @@ func (self *Whisper) add(envelope *Envelope) error {
 		// Notify the local node of a message arrival
 		go self.postEvent(envelope)
 	}
-	log.Trace(fmt.Sprintf("cached whisper envelope %x\n", envelope))
+	log.Trace(fmt.Sprintf("cached whisper envelope %x\n", hash))
 	return nil
 }