x86: Add NULL check to lsl (Jan Kiszka)

According to the Intel specs, lsl performs a check against NULL for the
provided selector, just like lar does. helper_lar() includes the
corresponding code, helper_lsl() was lacking it so far.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6863 c046a42c-6fe2-441c-8c8c-71466251a162