spice: fix memory leak

If errors happen for middle items of channel_list,
qmp_query_spice_channels() returns NULL, and the variable
cur_item going out of scope leaks the storage it points to.

The flag is a compatibility thing for older spice-server
versions. Meanwhile our minimum spice version requirement is
new enough that we should never ever see this error, and if we
do something went very seriously wrong. Let's using assert()
instead of returning NULL to avoid a memory leak.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>