[FIX] setup.nsi: fix the posgresql registry check:

My understanding is that EnumRegKey will index (the last argument)
the given registry path. When nothing is found, it sets the first
argument to the empty string. Only if the given path is wrong an
error is generated. So we have to go to the DoInstallPostgreSQL
label.

bzr revid: vmt@openerp.com-20130305110253-tu0t240liupxtchj