pgsql: Fix snapshot reference leak if lo_export fails.

Fix snapshot reference leak if lo_export fails.

If lo_export() fails to open the target file or to write to it, it leaks
the created LargeObjectDesc and its snapshot in the top-transaction
context and resource owner. That's pretty harmless, it's a small leak
after all, but it gives the user a "Snapshot reference leak" warning.

Fix by using a short-lived memory context and no resource owner for
transient LargeObjectDescs that are opened and closed within one function
call. The leak is easiest to reproduce with lo_export() on a directory
that doesn't exist, but in principle the other lo_* functions could also
fail.

Backpatch to all supported versions.

Reported-by: Andrew B
Reviewed-by: Alvaro Herrera
Discussion: https://www.postgresql.org/message-id/32bf767a-2d65-71c4-f170-122f416bab7e@iki.fi

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/8f32afee0179b2050bb1f84ba16fd1d4fba9a3b8

Modified Files
--------------
src/backend/libpq/be-fsstubs.c             | 147 ++++++++++++++---------------
src/backend/storage/large_object/inv_api.c |  36 ++++---
src/test/regress/input/largeobject.source  |  11 +++
src/test/regress/output/largeobject.source |  11 +++
4 files changed, 109 insertions(+), 96 deletions(-)