Correct memory fault when removing default nameref KSH_VERSION (#271)

This commit fixes a segmentation fault when an attempt was made to
unset the default KSH_VERSION variable prior any other nameref
activity such as creating another nameref or even reassigning the
nameref KSH_VERSION to something else.

(new shell without prior nameref activity)
$ nameref
KSH_VERSION=.sh.version
$ unset -n KSH_VERSION
Memory fault

src/cmd/ksh93/sh/name.c: _nv_unset():
- Add a 'Refdict' check before attempting to remove a value from it
  as apparently one does not exist until some sort of nameref
  activity occurs after shell startup as the default nameref of
  'KSH_VERSION=.sh.version' does not create one.

src/cmd/ksh93/tests/nameref.sh

@@ -684,4 +684,7 @@ typeset -n ref='arr[2]'
 
 $SHELL  2> /dev/null -c 'function x { nameref lv=gg ; compound -A lv.c=( [4]=( x=1 )) ; } ; compound gg ; x' || err_exit 'compound array assignment with nameref in a function failed'
 
+$SHELL -c 'unset -n KSH_VERSION' 2> /dev/null || err_exit 'Unable to unset nameref KSH_VERSION.'
+
+# ======
 exit $((Errors<125?Errors:125))