Fix #25

[niklas88]

Testing out my hypothesis that scanNonFunctionalRelation's special case for scanning the follow block should not read that from the psoFile it seems to fix the crash and return a correct result. Sadly I'm not sure how to trigger this in a small test query so I don't have a nice regression test.

My original argument was:

It seems to me that _psoFile is a WidthTwoList so pairs of Ids but we're trying to read a pair<Id, off_t> which is what indexFile consists of? Also the comments say we want to read a follow block but blocks are read from the indexFile