KVM: x86: fix use of uninitialized memory as segment descriptor in emulator.

If VMX reports segment as unusable, zero descriptor passed by the emulator
before returning. Such descriptor will be considered not present by the
emulator.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -4489,8 +4489,10 @@ static bool emulator_get_segment(struct x86_emulate_ctxt *ctxt, u16 *selector,
 	kvm_get_segment(emul_to_vcpu(ctxt), &var, seg);
 	*selector = var.selector;
 
-	if (var.unusable)
+	if (var.unusable) {
+		memset(desc, 0, sizeof(*desc));
 		return false;
+	}
 
 	if (var.g)
 		var.limit >>= 12;