hb_ot_map_builder_t::feature_info_t::cmp function returns 1 for equal arguments. This violates requirements to qsort's comparison function (which must be a total ordering) and causes UB at runtime. In practice this may mean strange runtime misbehaviors.

The issue can be easily fixed by prepending "a->seq == b->seq ? 0 :" to "a->seq < b->seq ? -1 : 1 to a->seq < b->seq ? -1 : 1".

This seems to be the only comparison function in HarfBazz which have such problems.