mark-sexp marks more than sexp within comment blocks

Example code with cursor as []:

```
  // []Start my application.  
  public func application(
```

Hit C-M-SPC, C-M-@ or `mark-sexp`, marked region in between []

```
  // [Start my application.  
  public] func application(
```

Expected (eg from objc-mode)
```
  // [Start] my application.  
  public func application(
```