Only a very basic logic bug in a _bt_insertonpg() caller could lead to a
violation of this invariant.  Besides, any newitemoff used for an
internal page is sanitized using other "can't happen" errors in
_bt_getstackbuf() or its callers, before _bt_insertonpg() even gets
called.

Also, move the error/assertion from the insert-without-split path of
_bt_insertonpg() to the top of the same function.  There is no reason
why this invariant only applies to insertions that happen to not result
in a page split; cover every insertion.  The assertion naturally belongs
next to the existing generic assertions that document relatively
high-level invariants for the item being inserted.