disas/i386: Avoid NULL pointer dereference in error case

In a code path where we hit an internal disassembler error, execution
would subsequently attempt to dereference a NULL pointer.  This
should never happen, but avoid the crash.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1488556233-31246-3-git-send-email-peter.maydell@linaro.org