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_sahf(struct x86_emulate_ctxt *ctxt)
{
	u32 flags;

	flags = EFLG_CF | EFLG_PF | EFLG_AF | EFLG_ZF | EFLG_SF;
	flags &= *reg_rmw(ctxt, VCPU_REGS_RAX) >> 8;

	ctxt->eflags &= ~0xffUL;
	ctxt->eflags |= flags | X86_EFLAGS_FIXED;
	return X86EMUL_CONTINUE;
}

static int em_sahf(struct x86_emulate_ctxt *ctxt)
{
	u32 flags;

	flags = EFLG_CF | EFLG_PF | EFLG_AF | EFLG_ZF | EFLG_SF;
	flags &= *reg_rmw(ctxt, VCPU_REGS_RAX) >> 8;

	ctxt->eflags &= ~0xffUL;
	ctxt->eflags |= flags | X86_EFLAGS_FIXED;
	return X86EMUL_CONTINUE;
}
