Fix bug in cancellation of non-exclusive backup to avoid assertion failure.

Previously an assertion failure occurred when pg_stop_backup() for
non-exclusive backup was aborted while it's waiting for WAL files to
be archived. This assertion failure happened in do_pg_abort_backup()
which was called when a non-exclusive backup was canceled.
do_pg_abort_backup() assumes that there is at least one non-exclusive
backup running when it's called. But pg_stop_backup() can be canceled
even after it marks the end of non-exclusive backup (e.g.,
during waiting for WAL archiving). This broke the assumption that
do_pg_abort_backup() relies on, and which caused an assertion failure.

This commit changes do_pg_abort_backup() so that it does nothing
when non-exclusive backup has been already marked as completed.
That is, the asssumption is also changed, and do_pg_abort_backup()
now can handle even the case where it's called when there is
no running backup.

Backpatch to 9.6 where SQL-callable non-exclusive backup was added.

Author: Masahiko Sawada and Michael Paquier
Reviewed-By: Robert Haas and Fujii Masao
Discussion: 
https://www.postgresql.org/message-id/CAD21AoD2L1Fu2c==gnvasmyfaaaq3y-aq2uevj-ztcgffjv...@mail.gmail.com

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/133d2fab2f83242ec99b15807a5920b48cc3d26c

Modified Files
--------------
src/backend/access/transam/xlog.c    | 36 ++++++++++++++++++++++++++++++++----
src/backend/replication/basebackup.c |  5 +++--
2 files changed, 35 insertions(+), 6 deletions(-)

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