Print WAL position correctly in pg_rewind error message.

This has been wrong ever since pg_rewind was added. The if-branch just
above this, where we print the same error with an extra message supplied
by XLogReadRecord() got this right, but the variable name was wrong in the
else-branch. As a consequence, the error printed the WAL position as
0/0 if there was an error reading a WAL file.

Backpatch to 9.5, where pg_rewind was added.

src/bin/pg_rewind/parsexlog.c

@@ -90,8 +90,7 @@ extractPageMap(const char *datadir, XLogRecPtr startpoint, int tliIndex,
 						 errormsg);
 			else
 				pg_fatal("could not read WAL record at %X/%X\n",
-						 (uint32) (startpoint >> 32),
-						 (uint32) (startpoint));
+						 (uint32) (errptr >> 32), (uint32) (errptr));
 		}
 
 		extractPageInfo(xlogreader);