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: 41061cdb
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>