Due to underfilled branch page. We're in the process of merging/moving
nodes to it because we already know it's underfilled. Took this approach
rather than just removing the assert in mdb_page_search_root, because
that assert may yet catch other situations we don't know about.
(Although, it has been there since the original commit of mdb.c and
has never triggered any other times...)
Howard Chu
568c22c4cc