pgsql: Fix error handling of XLogReaderAllocate in case of OOM

Fix error handling of XLogReaderAllocate in case of OOM

Similarly to previous fix 9b8d478, commit 2c03216 has switched
XLogReaderAllocate() to use a set of palloc calls instead of malloc,
causing any callers of this function to fail with an error instead of
receiving a NULL pointer in case of out-of-memory error. Fix this by
using palloc_extended with MCXT_ALLOC_NO_OOM that will safely return
NULL in case of an OOM.

Michael Paquier, slightly modified by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/6e4bf4ecd3c2a266870139462a079809dfe7ab8c

Modified Files
--------------
src/backend/access/transam/xlog.c | 7 +++++--
src/backend/access/transam/xlogreader.c | 30 ++++++++++++++++++++++-------
src/backend/replication/logical/logical.c | 5 +++++
src/bin/pg_rewind/parsexlog.c | 6 ++++++
4 files changed, 39 insertions(+), 9 deletions(-)