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

src/backend/access/transam/xlog.c

@@ -9523,13 +9523,13 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	if (XLogRecPtrIsInvalid(targetLSN))
 		return WALAVAIL_INVALID_LSN;
 
-	currpos = GetXLogWriteRecPtr();
-
 	/*
-	 * calculate the oldest segment currently reserved by all slots,
-	 * considering wal_keep_segments and max_slot_wal_keep_size
+	 * Calculate the oldest segment currently reserved by all slots,
+	 * considering wal_keep_segments and max_slot_wal_keep_size.  Initialize
+	 * oldestSlotSeg to the current segment.
 	 */
-	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+	currpos = GetXLogWriteRecPtr();
+	XLByteToSeg(currpos, oldestSlotSeg, wal_segment_size);
 	KeepLogSeg(currpos, &oldestSlotSeg);
 
 	/*
@@ -9548,6 +9548,9 @@ GetWALAvailability(XLogRecPtr targetLSN)
 	else
 		oldestSegMaxWalSize = 1;
 
+	/* the segment we care about */
+	XLByteToSeg(targetLSN, targetSeg, wal_segment_size);
+
 	/*
 	 * No point in returning reserved or extended status values if the
 	 * targetSeg is known to be lost.
@@ -9624,7 +9627,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	}
 
 	/* don't delete WAL segments newer than the calculated segment */
-	if (XLogRecPtrIsInvalid(*logSegNo) || segno < *logSegNo)
+	if (segno < *logSegNo)
 		*logSegNo = segno;
 }