pgsql: Fix uninitialized value in segno calculation

Fix uninitialized value in segno calculation

Remove previous hack in KeepLogSeg that added a case to deal with a
(badly represented) invalid segment number.  This was added for the sake
of GetWALAvailability.  But it's not needed if in that function we
initialize the segment number to be retreated to the currently being
written segment, so do that instead.

Per valgrind-running buildfarm member skink, and some sparc64 animals.

Discussion: https://postgr.es/m/1724648.1594230917@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b5b4c0fef9fb905d98f93c1f455397449c6e63a8

Modified Files
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src/backend/access/transam/xlog.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)