Robustify run-maude wrt killed *Maude* buffer

- for some reason, `comint-check-proc' returned non-NIL for a killed
  `inferior-maude-buffer'.

git-svn-id: https://maude-mode.svn.sourceforge.net/svnroot/maude-mode/trunk@62 468b7244-7632-0410-a5d6-f9412b4c873a