Result Cache, added in 9eacee2e neglected to properly adjust the plan
references in setrefs.c.  This could lead to the following error during
EXPLAIN:

ERROR:  cannot decompile join alias var in plan tree

Fix that.

Bug: 17030
Reported-by: Hans Buschmann
Discussion: https://postgr.es/m/17030-5844aecae42fe223@postgresql.org