Fix assertion failure in check_new_partition_bound().
Commit
6b2c4e59d was overly confident about not being able to see
a negative cmpval result from partition_range_bsearch(). Adjust
the code to cope with that.
Report and patch by Amul Sul; some additional cosmetic changes by me
Discussion: https://postgr.es/m/CAAJ_b97WCO=EyVA7fKzc86kKfojHXLU04_zs7-7+yVzm=-1QkQ@mail.gmail.com