move callback outside of try / catch

it's dangerous to callback inside try, because you may catch unexpected throws in callback.

lib/nconf/stores/file.js

@@ -97,12 +97,13 @@ File.prototype.load = function (callback) {
 
     try {
       self.store = self.format.parse(data.toString());
-      callback(null, self.store);
     }
     catch (ex) {
       self.store = {};
-      callback(ex);
+      return callback(ex);
     }
+    callback(null, self.store);
+
   });
 };