Small patch: fix code duplication in heapam.c

Hello

I discovered that there is a lot of code duplication in heapam.c.
In particular relation_openrv and relation_openrv_extended procedures
and also heap_openrv and heap_openrv_extended procedures are almost the
same. Here is a patch that fixes this.

-- 
Best regards,
Aleksander Alekseev
http://eax.me/