[RFC] vim-patch:7.4.251

```
Problem:  Crash when BufAdd autocommand wipes out the buffer.

Solution: Check for buffer to still be valid. Postpone freeing the
          buffer structure. (Hirohito Higashi)
```

https://code.google.com/p/vim/source/detail?r=29eb4c2a33ac701bfcd4d2e2bed7864eba876e0e