readline fix for rl_undo_list pointer aliasing; arith command sets word_top

tests/func.tests

@@ -166,6 +166,7 @@ export -f zf
 
 ${THIS_SH} -c 'type -t zf'
 ${THIS_SH} -c 'type zf'
+unset -f zf
 
 ${THIS_SH} ./func1.sub
 
@@ -179,6 +180,9 @@ ${THIS_SH} ./func3.sub
 # FUNCNEST testing
 ${THIS_SH} ./func4.sub
 
+# function naming restrictions
+${THIS_SH} ./func5.sub
+
 unset -f myfunction
 myfunction() {
     echo "bad shell function redirection"