nbtree: Move fastpath NULL descent stack assertion.

nbtree: Move fastpath NULL descent stack assertion.

Commit 074251db added an assertion that verified the fastpath/rightmost
page insert optimization's assumption about free space: There should
always be enough free space on the page to insert the new item without
splitting the page.  Otherwise, we end up using the "concurrent root
page split" phony/fake stack path in _bt_insert_parent().  This does not
lead to incorrect behavior, but it is likely to be far slower than
simply using the regular _bt_search() path.  The assertion catches
serious performance bugs that would probably take a long time to detect
any other way.

It seems much more natural to make this assertion just before the point
that we generate a fake/phony descent stack.  Move the assert there.
This also makes _bt_insertonpg() a bit more readable.