Workaround for linux w/tmp mounted as noexec; spellcheck fails (#2548)

js/background.js

@@ -192,6 +192,10 @@
     };
 
     const startSpellCheck = () => {
+      if (!window.enableSpellCheck || !window.disableSpellCheck) {
+        return;
+      }
+
       if (window.Events.getSpellCheck()) {
         window.enableSpellCheck();
       } else {