0.9.7.30: robustify sb-introspect when used with extended function names

0.9.7.30: robustify sb-introspect when used with extended function names
 ...so that (find-definition-sources-by-name '(setf foo) :class) will
    return NIL instead of signaling an error.
 ...this makes it simpler to support M-. for SETF-names in Slime.