Avoid potential UCollator leak for older ICU versions.

Avoid potential UCollator leak for older ICU versions.

ICU versions 53 and earlier rely on icu_set_collation_attributes() to
process the attributes in the locale string. Avoid leaking the
already-opened UCollator object if an error is encountered.

Discussion: https://postgr.es/m/04182066-7655-344a-b8b7-040b1b2490fb%40enterprisedb.com
Reviewed-by: Peter Eisentraut