[FIX] web_editor: remove destroyed editors from memory

Commit https://github.com/odoo/odoo/commit/f6bbc40fbe16ea5ab01ad03b8e1519c4e559bf6d
introduced a method to destroy all current editors to refresh them when
necessary... but forgot to empty the array that contains them.

In a 12.0 without custo, this is only slowing down the editor but in
master (or with custo) this makes the editor crash.

closes odoo/odoo#40372

X-original-commit: 2b5661937d9772b98c73f9f64aa419fec7676052
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -1103,6 +1103,7 @@ var SnippetsMenu = Widget.extend({
         _.each(this.snippetEditors, function (snippetEditor) {
             snippetEditor.destroy();
         });
+        this.snippetEditors.splice(0);
     },
     /**
      * Calls a given callback 'on' the given snippet and all its child ones if