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