whisper: fix empty topic (#15811)

* whisper: fix empty topic

* whisper: add check to matchSingleTopic

* whisper: add tests

* whisper: fix gosimple

* whisper: added lastTopicByte const

1 files changed, 6 insertions, 2 deletions

diff --git a/whisper/whisperv6/filter.go b/whisper/whisperv6/filter.go
index 2f52dd6b9..a752c7ac9 100644
--- a/whisper/whisperv6/filter.go
+++ b/whisper/whisperv6/filter.go
@@ -221,8 +221,12 @@ func (f *Filter) MatchTopic(topic TopicType) bool {
 }
 
 func matchSingleTopic(topic TopicType, bt []byte) bool {
-	if len(bt) > 4 {
-		bt = bt[:4]
+	if len(bt) > TopicLength {
+		bt = bt[:TopicLength]
+	}
+
+	if len(bt) < TopicLength {
+		return false
 	}
 
 	for j, b := range bt {