:Rvm with a buffer of a non-existent file generates error.

CtrlpMRUFiles opens also non-existent files, but rvm (the cmd line tootl) doesn't like this. I've put a check to avoid this.

I've in my .vimrc

```vim
augroup rvm
  autocmd!
  autocmd BufEnter * Rvm
augroup END
```