Fix channels showing up across teams when multiple teams open in same browser (#3329)

api/channel.go

@@ -784,6 +784,11 @@ func getChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		member := cmresult.Data.(model.ChannelMember)
 		data.Member = &member
 
+		if data.Channel.TeamId != c.TeamId {
+			c.Err = model.NewLocAppError("getChannel", "api.channel.get_channel.wrong_team.app_error", map[string]interface{}{"ChannelId": id, "TeamId": c.TeamId}, "")
+			return
+		}
+
 		if HandleEtag(data.Etag(), w, r) {
 			return
 		} else {