Fix incautious CTE matching in rewriteSearchAndCycle().

Fix incautious CTE matching in rewriteSearchAndCycle().

This function looks for a reference to the recursive WITH CTE,
but it checked only the CTE name not ctelevelsup, so that it could
seize on a lower CTE that happened to have the same name.  This
would result in planner failures later, either weird errors such as
"could not find attribute 2 in subquery targetlist", or crashes
or assertion failures.  The code also merely Assert'ed that it found
a matching entry, which is not guaranteed at all by the parser.

Per bugs #17320 and #17318 from Zhiyong Wu.
Thanks to Kyotaro Horiguchi for investigation.

Discussion: https://postgr.es/m/17320-70e37868182512ab@postgresql.org
Discussion: https://postgr.es/m/17318-2eb65a3a611d2368@postgresql.org