KVM: x86: zero base3 of unusable segments

[ Upstream commit f0367ee ]

Static checker noticed that base3 could be used uninitialized if the
segment was not present (useable).  Random stack values probably would
not pass VMCS entry checks.

Reported-by:  Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 1aa3661 ("KVM: x86 emulator: consolidate segment accessors")
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

arch/x86/kvm/x86.c

@@ -4910,6 +4910,8 @@ static bool emulator_get_segment(struct x86_emulate_ctxt *ctxt, u16 *selector,
 
 	if (var.unusable) {
 		memset(desc, 0, sizeof(*desc));
+		if (base3)
+			*base3 = 0;
 		return false;
 	}