Plug memory leak in index_get_partition

The list of indexes was being leaked when asked for an index that
doesn't have an index partition in the table partition.  Not a common
case admittedly --and in most cases where it occurs, caller throws an
error anyway-- but worth fixing for cleanliness and in case any
third-party code is calling this function.

While at it, remove use of lfirst_oid() to obtain a value we already
have.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20201105203606.GF22691@telsasoft.com
