x86, tls: read no more than saved TLS entries

  While writing this, I somehow managed to miss the check of
how many entries were saved in core image.
So it may dereference here bs.

Fixes: #228
Fixes: commit 6fde3b8c27db ("x86: restore TLS")

travis-ci: success for x86, tls: read no more than saved TLS entries
Cc: Andrei Vagin <avagin@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>