KVM: emulate: avoid accessing NULL ctxt->memopp

A failure to decode the instruction can cause a NULL pointer access.
This is fixed simply by moving the "done" label as close as possible
to the return.

This fixes CVE-2014-8481.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Cc: stable@vger.kernel.org
Fixes: 41061cdb98a0bec464278b4db8e894a3121671f5
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
static int em_loop(struct x86_emulate_ctxt *ctxt)
{
	int rc = X86EMUL_CONTINUE;

	register_address_increment(ctxt, reg_rmw(ctxt, VCPU_REGS_RCX), -1);
	if ((address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) != 0) &&
	    (ctxt->b == 0xe2 || test_cc(ctxt->b ^ 0x5, ctxt->eflags)))
		rc = jmp_rel(ctxt, ctxt->src.val);

	return rc;
}

static int em_loop(struct x86_emulate_ctxt *ctxt)
{
	int rc = X86EMUL_CONTINUE;

	register_address_increment(ctxt, reg_rmw(ctxt, VCPU_REGS_RCX), -1);
	if ((address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) != 0) &&
	    (ctxt->b == 0xe2 || test_cc(ctxt->b ^ 0x5, ctxt->eflags)))
		rc = jmp_rel(ctxt, ctxt->src.val);

	return rc;
}
