Handle bogus %cs selector in single-step instruction decoding (CVE-2007-3731)

The code for LDT segment selectors was not robust in the face of a bogus
selector set in %cs via ptrace before the single-step was done.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>