Hi,

I got an error when try to revert a buffer:
vc-revert: Wrong type argument: stringp, nil

Here is the backtrace:
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  vc-buffer-sync(nil)
  vc-revert()
  funcall-interactively(vc-revert)
  command-execute(vc-revert)


It caused by reverting a buffer which has no file name (like create a buffer but not saving it).
 
This patch will check the `` bebuffer-file-name` before call subsequent function.
Please help review the patch, thanks

Best Regards
Lin