pg_upgrade: Upgrade an Assert to a real 'if' test

It seems possible for the condition being tested to be true in
production, and nobody would never know (except when some data
eventually becomes corrupt?).

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m//202109040001.zky3wgv2qeqg@alvherre.pgsql

src/bin/pg_rewind/parsexlog.c

@@ -102,7 +102,9 @@ extractPageMap(const char *datadir, XLogRecPtr startpoint, int tliIndex,
 	 * If 'endpoint' didn't point exactly at a record boundary, the caller
 	 * messed up.
 	 */
-	Assert(xlogreader->EndRecPtr == endpoint);
+	if (xlogreader->EndRecPtr != endpoint)
+		pg_fatal("end pointer %X/%X is not a valid end point; expected %X/%X",
+				 LSN_FORMAT_ARGS(endpoint), LSN_FORMAT_ARGS(xlogreader->EndRecPtr));
 
 	XLogReaderFree(xlogreader);
 	if (xlogreadfd != -1)