Fix longstanding error in _bt_search(): should moveright at top of loop not

Fix longstanding error in _bt_search(): should moveright at top of loop not
bottom.  Otherwise we fail to moveright when the root page was split while
we were "in flight" to it.  This is not a significant problem when the root
is above the leaf level, but if the root was also a leaf (ie, a single-page
index just got split) we may return the wrong leaf page to the caller,
resulting in failure to find a key that is in fact present.  Bug has existed
at least since 7.1, probably forever.