Fix potential NULL pointer dereference in ITS code
Reported by Shashank in: <https://savannah.gnu.org/bugs/?63529> and
<https://savannah.gnu.org/bugs/?63530>.
* gettext-tools/src/locating-rule.c (get_attribute): Check return
value of xmlGetProp.
(document_locating_rule_match): Check return value of
xmlDocGetRootElement.
(locating_rule_list_add_from_file): Likewise.
Signed-off-by: Daiki Ueno <ueno@gnu.org>
