Fix possibility of creating a "phantom" segment after promotion.

When promoting a standby just after a XLOG_SWITCH record was replayed,
and next segment(s) are already are locally available (via walsender,
restore_command + trigger/recovery target), that segment could
accidentally be recycled onto the past of the new timeline.  Later
checkpointer would create a .ready file for it, assuming there was an
error during creation, and it would get archived.  That causes trouble
if another standby is later brought up from a basebackup from before
the timeline creation, because it would try to read the
segment, because XLogFileReadAnyTLI just tries all possible timelines,
which doesn't have valid contents.  Thus replay would fail.

The problem, if already occurred, can be fixed by removing the segment
and/or having restore_command filter it out.

The reason for the creation of such "phantom" segments was, that after
an XLOG_SWITCH record the EndOfLog variable points to the beginning of
the next segment, and RemoveXlogFile() used XLByteToPrevSeg().
Normally RemoveXlogFile() doing so is harmless, because the last
segment will still exist preventing InstallXLogFileSegment() from
causing harm, but just after promotion there's no previous segment on
the new timeline.

Fix that by using XLByteToSeg() instead of XLByteToPrevSeg().

Author: Andres Freund
Reported-By: Greg Burek
Discussion: https://postgr.es/m/20170619073026.zcwpe6mydsaz5ygd@alap3.anarazel.de
Backpatch: 9.2-, bug older than all supported versions

src/backend/access/transam/xlog.c

@@ -3974,7 +3974,7 @@ RemoveXlogFile(const char *segname, XLogRecPtr PriorRedoPtr, XLogRecPtr endptr)
 	/*
 	 * Initialize info about where to try to recycle to.
 	 */
-	XLByteToPrevSeg(endptr, endlogSegNo);
+	XLByteToSeg(endptr, endlogSegNo);
 	if (PriorRedoPtr == InvalidXLogRecPtr)
 		recycleSegNo = endlogSegNo + 10;
 	else