monitor: Fix missing err = NULL in client_migrate_info()

When SPICE isn't used, we either fail an assertion in error_set(),
or leak an error object.  Broken in commit b25d81b.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
