When doing spill-to-disk on a temporary table used for recursive CTE,
we move from MEMORY to InnoDB and thus need to reposition our cursor
at the right row. However, if the table was never opened for reading in
the first place (because the spill-to-disk happened while we were still
writing nonrecursive rows), we would hit an InnoDB assert error.

Fix by simply not trying to reposition the cursor if we haven't read
anything yet.

Change-Id: I21b47ce8a24af997d60b327b3faa09aa4b6a4139