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?