Avoid dividing by zero

This led to mysterious behaviour when testing buggy Verlet kernels,
because the division silently caused execution to drop straight to the

map { eval $_ } @work

construct at the end. This script must die! :-)

Change-Id: I6da2dcc7b96b6815890820f0b24361d7c511f8a8
Merge "Avoid dividing by zero"