wlist.c, wl_find(), avoid crash when `string' argument is NULL

src/misc/wlist.c

@@ -347,6 +347,9 @@ wl_chop_rest(wordlist *wl)
 wordlist *
 wl_find(const char *string, const wordlist *wl)
 {
+    if (!string)
+        return NULL;
+
     for (; wl; wl = wl->wl_next)
         if (eq(string, wl->wl_word))
             break;