If client_migrate_info was called once with
cert-host-subject and then again without
cert-host-subject, on a third call to
client_migrate info, the cert-host-subject from the first
call would have been freed for the second time.
[ An even more detailed explanation:
In the first call with cert-host-subject a pointer was
In the second call without cert-host-subject the pointer
was freed, but not set to NULL.
In the third call that same pointer was freed again.
glibc complains about double free and later it causes
a segmentation fault.
Set the pointer to NULL, if cert-host-subject is not given.
[ That's fix happens in the "second" migrate-info ]
The pointer is freed only once and qemu-kvm/spice-server does not crash.