btr_cur_pessimistic_insert(): Relax a too strict debug assertion that
would fail when the function is invoked by btr_cur_pessimistic_update()
during innobase_add_instant_try(), that is, when updating the hidden
metadata record during a subsequent ADD COLUMN operation involves
splitting the leftmost clustered index leaf page.

This is a partial backport of 301bd62b
from 10.4.