[FIX] web_tip: destroy method set call off() on undefined

In some cases, when the destoy method of 'Tip' calls the off() function on
'$breathing', even if this one is not defined.

To fix this error, we check that '$breathing' is not 'undefined' before
calling off() on it.

addons/web_tip/static/src/js/tip.js

@@ -300,6 +300,8 @@ var Tip = Class.extend({
         if (this.$element) {
             this.$element.popover('destroy');
             this.$element.removeClass('oe_tip_show_element');
+        }
+        if (this.$breathing) {
             this.$breathing.off().remove();
         }
         if (this.$helper) {