Avoid Nans in primary vertex

While investigating the irreproducibility of relvals I discovered that  in some cases no PixelVertices were reconstructed. I trace this down to some Nans.
Here I try to trap Nans and conditions that will generate Nans.
The effect on OfflineVertices is essentially negligible while evident to PixelVertices.
This is also one of the reason that prompted #5476 

Few more optimizations have been added to the algorithms.
AdaptiveVertexFitter and KalmanVertexFitter (and the infrastructure used in there) would benefit of a complete re-engineering in light of C++11 and vectorization.


@mtosi, could you please test this for HLT?
sure many things can be simplified, thanks for spotting (and this code was there since ages and it si now essentially unmantained)
this is only at initialization though