Fix memory leak in printtup.c.
Fix memory leak in printtup.c.

Commit f2dec34e1 changed things so that printtup's output stringinfo
buffer was allocated outside the per-row temporary context, not inside
it.  This creates a need to free that buffer explicitly when the temp
context is freed, but that was overlooked.  In most cases, this is all
happening inside a portal or executor context that will go away shortly
anyhow, but that's not always true.  Notably, the stringinfo ends up
getting leaked when JDBC uses row-at-a-time fetches.  For a query
that returns wide rows, that adds up after awhile.

Per bug #15700 from Matthias Otterbach.  Back-patch to v11 where the
faulty code was added.

Discussion: 1edb65c6cd3a1c7c0db1e49a7a7b292e57e60b31@postgresql.org

greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb