vlc_strcasestr(): fix infinite loop an invalid haystack

Pointed-out-by: Edward Wang <edward.c.wang@compdigitec.com>
(cherry picked from commit baada5ed)

src/text/unicode.c

@@ -336,7 +336,7 @@ char *vlc_strcasestr (const char *haystack, const char *needle)
         s = vlc_towc (haystack, &(uint32_t) { 0 });
         haystack += s;
     }
-    while (s != 0);
+    while (s > 0);
 
     return NULL;
 }