target/i386/cpu: Fix memleak in x86_cpu_class_check_missing_features

'err' forgot to free in x86_cpu_class_check_missing_features error path.
Fix that.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200831134315.1221-7-pannengyuan@huawei.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>