os: Check for integer overflow on BigRequest length

Check for another possible integer overflow once we get a complete xReq
with BigRequest.

Related to CVE-2025-49176

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Peter Harris <pharris2@rocketsoftware.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2028>
(cherry picked from commit 4fc4d76b2c)

os/io.c

@@ -390,6 +390,8 @@ ReadRequestFromClient(ClientPtr client)
                     needed = get_big_req_len(request, client);
             }
             client->req_len = needed;
+            if (needed > MAXINT >> 2)
+                return -(BadLength);
             needed <<= 2;
         }
         if (gotnow < needed) {