Avoid a possible out-of-bounds access (src/backend/optimizer/util/relnode.c)

Hi,

Per Coverity.
CID 1518088 (#2 of 2): Improper use of negative value (NEGATIVE_RETURNS)

The function bms_singleton_member can returns a negative number.

/*
* Get a child rel for rel2 with the relids. See above comments.
*/
if (rel2_is_simple)
{
int varno = bms_singleton_member(child_relids2);

child_rel2 = find_base_rel(root, varno);
}

It turns out that in the get_matching_part_pairs function (joinrels.c), the
return of bms_singleton_member is passed to the find_base_rel function,
which cannot receive a negative value.

find_base_rel is protected by an Assertion, which effectively indicates
that the error does not occur in tests and in DEBUG mode.

But this does not change the fact that bms_singleton_member can return a
negative value, which may occur on some production servers.

Fix by changing the Assertion into a real test, to protect the
simple_rel_array array.

best regards,
Ranier Vilela