We check whether the variable machine is NULL or not before accessing
it. If machine is NULL, exit QEMU with an error, this can avoids a
segfault error.
Markus Armbruster <armbru@redhat.com> adds that the segfault can be
reproduced as follows:
$ qemu-system-xtensa -cpu help
Signed-off-by: Dunrong Huang <riegamaths@gmail.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>