fix for number of secondary vertex tracks in case IVF vertices are used

This fix does not affect the any b-tagger in the RECO sequence, it only affects the performance when IVF is used and even in that case the performance is very similar, as can be seen in the figure.
![performance_clean](https://cloud.githubusercontent.com/assets/5425769/4075262/2acb4102-2ead-11e4-994f-8a1ee79a0605.png)