Avoid assert

Avoid assert

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...)