pgsql: Fix freeing old values in index_store_float8_orderby_distances()

Fix freeing old values in index_store_float8_orderby_distances()

6cae9d2c10 has added an error in freeing old values in
index_store_float8_orderby_distances() function. It looks for old value in
scan->xs_orderbynulls[i] after setting a new value there.
This commit fixes that. Also it removes short-circuit in handling
distances == NULL situation. Now distances == NULL will be treated the same
way as array with all null distances. That is, previous values will be freed
if any.

Reported-by: Tom Lane, Nikita Glukhov
Discussion: https://postgr.es/m/CAPpHfdu2wcoAVAm3Ek66rP%3Duo_C-D84%2B%2Buf1VEcbyi_caBXWCA%40mail.gmail.com
Discussion: https://postgr.es/m/426580d3-a668-b9d1-7b8e-f74d1a6524e0%40postgrespro.ru
Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8c8a267201bebb0edeaab2a76b7d3bcc9739924f

Modified Files
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src/backend/access/index/indexam.c | 40 +++++++++++++++++++-------------------
1 file changed, 20 insertions(+), 20 deletions(-)