x86 ptrace: disallow null cs/ss

In my revamp of the x86 ptrace code for setting register values,
I accidentally omitted a check that was there in the old code.
Allowing %cs to be 0 causes a bad crash in recovery from iret failure.
This patch fixes that regression against 2.6.24, and adds a comment
that should help prevent this subtlety from being overlooked again.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>