Fix incorrect lack of Datum conversion in _int_matchsel()

Fix incorrect lack of Datum conversion in _int_matchsel()

The code used

    return (Selectivity) 0.0;

where

    PG_RETURN_FLOAT8(0.0);

would be correct.

On 64-bit systems, these are pretty much equivalent, but on 32-bit
systems, PG_RETURN_FLOAT8() correctly produces a pointer, but the old
wrong code would return a null pointer, possibly leading to a crash
elsewhere.

We think this code is actually not reachable because bqarr_in won't
accept an empty query, and there is no other function that will
create query_int values.  But better be safe and not let such
incorrect code lie around.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/8246d7ff-f4b7-4363-913e-827dadfeb145%40eisentraut.org