[FIX] mail: prevent pos tour from sometimes failing

Here is what could (sometimes) happen in community:
- the point of sale tour is started by the runbot
- it loads the '/web' url
- this instantiate the mail client action (init)
- the tour sees that the POS link is in the dom and clicks on it
- the action manager destroys the discuss client action (before its
  start method is called)
- the destroy method from the client action is called, this.$buttons
  is not set, the destroy method causes a traceback

addons/mail/static/src/js/client_action.js

@@ -660,7 +660,9 @@ var ChatAction = Widget.extend(ControlPanelMixin, {
         });
     },
     destroy: function() {
-        this.$buttons.off().destroy();
+        if (this.$buttons) {
+            this.$buttons.off().destroy();
+        }
         this._super.apply(this, arguments);
     },