Fixes issue with vsiten and Verlet buffers

Commit 0336ab2d only fixed part of the vsiten issue in the Verlet
buffer calculation code. Parameters were read from incorrect memory
locations, which could lead to a segmentation fault or incorrect
masses for vsiten particles. It unlikely that this affected results.
Part of #1579.

Change-Id: I76cdb94e34194d2f6d49d98a49486ce1df76d91a
Merge release-4-6 into release-5-0

Change-Id: I08dc9d5e776480e7686a159cfcd981ba46c510ad