lexical #3/19, attach and instantly remove a NULL wl_word here

this modification is faithful iff cw != NULL  and  cw->wl_next == NULL
  cw != NULL because we have just survived an update of cw->wl_word
  cw->wl_next == NULL is witnessed by:
    1) cw either NULL or pointing to a wl_cons(,NULL)
    2) cw not being modified, and no wl_next being modified
    3) check of all function invocations with wlist/cw argument
         not to modify any wl_next slot
which are:
  1) cp_ccom(wlist, ...)
  2) wl_free() always followed by goto nloop, which does a cw = wl_cons(,NULL)

and cp_ccom() doesn't modify any wl_next slot

the first part of the patch is intentionally similar to newword
`i' and `buf' can be modified here, because we will inevitably `goto done'
attach and instantly remove a NULL wl_word here

src/frontend/parser/lexical.c

@@ -185,6 +185,18 @@ gotchar:
             if (i) {
                 buf[i] = '\0';
                 cw->wl_word = copy(buf);
+                // assumption: cw != NULL and cw->wl_next == NULL
+                cw->wl_next = wl_cons(NULL, NULL);
+                cw->wl_next->wl_prev = cw;
+                cw = cw->wl_next;
+                // note: we can overwrite buf and i here
+                //   they  won't be accessed any more (see `goto done')
+                bzero(buf, NEW_BSIZE_SP);
+                i = 0;
+                // cw->wl_prev is the recent cw
+                //   thus this wl_next has been NULL per assumtion vrom above
+                cw->wl_prev->wl_next = NULL;
+                tfree(cw);
             } else  {
                 if (cw->wl_prev) {
                     cw->wl_prev->wl_next = NULL;