Fix scanning a table type into a struct

Table types have system / hidden columns like tableoid, cmax, xmax, etc.
These are not included when sending or receiving composite types.

https://github.com/jackc/pgx/issues/1576

conn.go

@@ -1282,7 +1282,9 @@ func (c *Conn) getCompositeFields(ctx context.Context, oid uint32) ([]pgtype.Com
 	var fieldOID uint32
 	rows, _ := c.Query(ctx, `select attname, atttypid
 from pg_attribute
-where attrelid=$1 and not attisdropped
+where attrelid=$1
+	and not attisdropped
+	and attnum > 0
 order by attnum`,
 		typrelid,
 	)