[STUMP] Crash in UTF8-TO-STRING with CLISP

I've received the following bug report from a Debian user:

  In my case I have the following in my ~/.emacs:

      (setq-default frame-title-format (list "" "Emacs Macht Alle Computer Sch\366n"))

  Eval this in an Emacs buffer an watch stumpwm crash:

      *** - CONVERT-STRING-FROM-BYTES: Invalid byte sequence #xF6 #x6E #x00 #xA0 in CHARSET:UTF-8 conversion

The given frame title is not properly encoded, but it shouldn't crash
Stumpwm.  I can see utf8-to-string function in wrappers.lisp handles
coding errors for SBCL.  Would it be possible to handle them for CLISP
as well?