regression introduced in commit r7280 on Feb 5 2013: "(install_logger)
unify with callback_uninst(), thus fixing counting for erasures"

(spot by Colin on installing a minimal system with urpmi)

callback_open got called one too much (it got called twice for first
package which is actually filesystem), thus we bump the index one too
much.

the call chain is:
rpmtsRun() -> runTransScripts() -> rpmteProcess() which says:
    /* Dont bother opening for elements without pre/posttrans scripts */

But since we now have package ('filesystem') with %pretrans due to /usr
migration, it goes further and calls rpmteOpen()->rpmteFDHeader() which
notifys us about a package opening in order to get its file descriptor.