xkb: proof GetCountedString against request length attacks

GetCountedString did a check for the whole string to be within the
request buffer but not for the initial 2 bytes that contain the length
field. A swapped client could send a malformed request to trigger a
swaps() on those bytes, writing into random memory.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 11beef0b7f)

xkb/xkb.c

@@ -5138,6 +5138,11 @@ _GetCountedString(char **wire_inout, ClientPtr client, char **str)
     CARD16 len;
 
     wire = *wire_inout;
+
+    if (client->req_len <
+        bytes_to_int32(wire + 2 - (char *) client->requestBuffer))
+        return BadValue;
+
     len = *(CARD16 *) wire;
     if (client->swapped) {
         swaps(&len);