pgsql: Introduce XLogRecPtrIsValid()

Introduce XLogRecPtrIsValid()

XLogRecPtrIsInvalid() is inconsistent with the affirmative form of
macros used for other datatypes, and leads to awkward double negatives
in a few places.  This commit introduces XLogRecPtrIsValid(), which
allows code to be written more naturally.

This patch only adds the new macro.  XLogRecPtrIsInvalid() is left in
place, and all existing callers remain untouched.  This means all
supported branches can accept hypothetical bug fixes that use the new
macro, and at the same time any code that compiled with the original
formulation will continue to silently compile just fine.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/aQB7EvGqrbZXrMlg@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/49b45999f3b213c00cae2ef4f42d56e88c96b709

Modified Files
--------------
src/include/access/xlogdefs.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)