Make sure that an empty string never matches a listbox entry in
LISTBOX_FindString().

controls/listbox.c

diff --git a/controls/listbox.c b/controls/listbox.c
index c08dd8a..4cab69e 100644
--- a/controls/listbox.c
+++ b/controls/listbox.c
@@ -774,7 +774,7 @@
     item = descr->items + start + 1;
     if (HAS_STRINGS(descr))
     {
-        if (!str) return LB_ERR;
+        if (!str || ! str[0] ) return LB_ERR;
         if (exact)
         {
             for (i = start + 1; i < descr->nb_items; i++, item++)