Fix panic on WebhookURL only setting (mattermost). Closes #916 (#917)

bridge/mattermost/handlers.go

@@ -66,6 +66,11 @@ func (b *Bmattermost) handleMatter() {
 		} else {
 			b.Log.Debugf("Choosing login/password based receiving")
 		}
+		// if for some reason we only want to sent stuff to mattermost but not receive, return
+		if b.GetString("WebhookBindAddress") == "" && b.GetString("WebhookURL") != "" {
+			b.Log.Debugf("No WebhookBindAddress specified, only WebhookURL. You will not receive messages from mattermost, only sending is possible.")
+			return
+		}
 		go b.handleMatterClient(messages)
 	}
 	var ok bool