bug#2284: same problem in grep-mode

[Peter Feigl]

probably because this mode also uses compilation-mode

Debugger entered--Lisp error: (wrong-type-argument window-live-p nil)
  select-window(nil nil)
  pop-to-buffer(#<buffer icpcfaKQgJas_.s> other-window)
  compilation-goto-locus(#<marker at 514 in *grep*> #<marker at 282 in 
icpcfaKQgJas_.s> #<marker at 283 in icpcfaKQgJas_.s>)
  compilation-next-error-function(0 nil)
  next-error-internal()
  compile-goto-error(13)
  call-interactively(compile-goto-error nil nil)