Prevent generation of bogus subquery scan paths.

Prevent generation of bogus subquery scan paths.

Commit 0927d2f46ddd4cf7d6bf2cc84b3be923e0aedc52 didn't check that
consider_parallel was set for the target relation or account for
the possibility that required_outer might be non-empty.

To prevent future bugs of this ilk, add some assertions to
add_partial_path and do a bit of future-proofing of the code
recently added to recurse_set_operations.

Report by Andreas Seltenreich.  Patch by Jeevan Chalke.  Review
by Amit Kapila and by me.

Discussion: http://postgr.es/m/CAM2+6=U+9otsyF2fYB8x_2TBeHTR90itarqW=qAEjN-kHaC7kw@mail.gmail.com