whisper: fix spurious From identity with untargeted messages

whisper/whisper.go

@@ -260,9 +260,11 @@ func (self *Whisper) open(envelope *Envelope) *Message {
 	// Iterate over the keys and try to decrypt the message
 	for _, key := range self.keys {
 		message, err := envelope.Open(key)
-		if err == nil || err == ecies.ErrInvalidPublicKey {
+		if err == nil {
 			message.To = &key.PublicKey
 			return message
+		} else if err == ecies.ErrInvalidPublicKey {
+			return message
 		}
 	}
 	// Failed to decrypt, don't return anything