xmlschemastypes.c: xmlSchemaGetFacetValueAsULong add, check "facet->val" The xmlSchemaGetFacetValueAsUlong() API is an external API. The validity of external input parameters must be strictly verified. Before accessing "facet->val->value", we need check whether "facet->val" is a null pointer. Signed-off-by: wuqing <wuqing30@huawei.com> Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>