In the code as was, the histogram bins corresponding to the vertex were only
zereod to prevent the vertex being found again, if multiple vertices were requested.
So if requireing only 1 vertex, if the significance of the found vertex was not high
enough it would not have been added, so the check on the number of found vertex
candidates would not change, and we would be doomed to find this vertex for ever.

With a lower significance the vertex would have been added, and with more required
vertices, the histogram bins would have been zeroed.

As such, have added checks such that the bins are *always* zeroed, and also, that
if the vertex is not significant, then it bombs out of the loop, since no subsequent
vertex could have a higher significance. Either of these changes should by themselves,
be enough to prevent this happening.

This addresses ATR-19265.