xkb: length-check XkbListComponents before accessing the fields

Each string length field was accessed before checking whether that byte
was actually part of the client request. No real harm here since it
would immediately fail with BadLength anyway, but let's be correct here.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

xkb/xkb.c

@@ -5870,6 +5870,8 @@ ProcXkbListComponents(ClientPtr client)
      * length wrong. */
     str = (unsigned char *) &stuff[1];
     for (i = 0; i < 6; i++) {
+        if (!_XkbCheckRequestBounds(client, stuff, str, str + 1))
+            return BadLength;
         size = *((uint8_t *)str);
         len = (str + size + 1) - ((unsigned char *) stuff);
         if ((XkbPaddedSize(len) / 4) > stuff->length)