db: don't enter into db_upgrades table on every startup.

Below this code appears:

	if (current != orig)
		db_exec(__func__, db,
			"INSERT INTO db_upgrades VALUES (%i, '%s');",
			orig, version());

But since the loop pre-increments current, this is always true.  I wondered
why there were so many duplicates in my db_upgrades table!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

wallet/db.c

@@ -636,8 +636,8 @@ static void db_migrate(struct db *db, struct log *log)
 		log_info(log, "Updating database from version %u to %u",
 			 current, available);
 
-	while (++current <= available)
+	while (current < available)
-		db_exec(__func__, db, "%s", dbmigrations[current]);
+		db_exec(__func__, db, "%s", dbmigrations[++current]);
 
 	/* Finally update the version number in the version table */
 	db_exec(__func__, db, "UPDATE version SET version=%d;", available);