Fix minor bug in XLogFileRead() that accidentally worked.

Cascading replication copied the incoming file into pg_xlog but
didn't set path correctly, so the first attempt to open file failed
causing it to loop around and look for file in pg_xlog. So the
earlier coding worked, but accidentally rather than by design.

Spotted by Fujii Masao, fix by Fujii Masao and Simon Riggs

src/backend/access/transam/xlog.c

@@ -2805,6 +2805,11 @@ XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
 					 errmsg("could not rename file \"%s\" to \"%s\": %m",
 							path, xlogfpath)));
 
+		/*
+		 * Set path to point at the new file in pg_xlog.
+		 */
+		strncpy(path, xlogfpath, MAXPGPATH);
+
 		/*
 		 * If the existing segment was replaced, since walsenders might have
 		 * it open, request them to reload a currently-open segment.