fix potential race condition when en-/dis-abling E2EE

modules/e2ee/KeyHandler.js

      * @returns {void}
      */
     async setEnabled(enabled) {
+        this._enabling && await this._enabling;
+
         if (enabled === this.enabled) {
             return;
         }
 
-        this._enabling && await this._enabling;
-
         this._enabling = new Deferred();
 
         this.enabled = enabled;