perf: A nitpick, make it better. (#273)

Author: Runrioter

lib/context.js

@@ -70,7 +70,7 @@ module.exports = function ctx(compiler, options) {
 
     // We are now in invalid state
     context.state = false;
-    if (callback && typeof callback === 'function') {
+    if (typeof callback === 'function') {
       callback();
     }
   }