[FAB-14475] fix data race in gossip/channel

In gossip/channel.GenerateMAC()
create a new slice instead of maybe appending to existing slice

Change-Id: I2e88b87d88ddccf5e3c45471cc07656acd7e7af6
Signed-off-by: Hagar Meir <hagar.meir@ibm.com>
(cherry picked from commit 435d4c4)

gossip/gossip/channel/channel.go

@@ -956,6 +956,8 @@ func (cache *stateInfoCache) Stop() {
 // and a channel name
 func GenerateMAC(pkiID common.PKIidType, channelID common.ChainID) []byte {
 	// Hash is computed on (PKI-ID || channel ID)
-	preImage := append([]byte(pkiID), []byte(channelID)...)
+	var preImage []byte
+	preImage = append(preImage, []byte(pkiID)...)
+	preImage = append(preImage, []byte(channelID)...)
 	return common_utils.ComputeSHA256(preImage)
 }