Prevent generation of bogus subquery scan paths.

Commit 0927d2f46d 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

src/backend/optimizer/util/pathnode.c

@@ -770,6 +770,12 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 	/* Check for query cancel. */
 	CHECK_FOR_INTERRUPTS();
 
+	/* Path to be added must be parallel safe. */
+	Assert(new_path->parallel_safe);
+
+	/* Relation should be OK for parallelism, too. */
+	Assert(parent_rel->consider_parallel);
+
 	/*
 	 * As in add_path, throw out any paths which are dominated by the new
 	 * path, but throw out the new path if some existing path dominates it.