Fix coverage broken by #2016 and #2434.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19974 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2016 - fix another bad use of cells vector

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19975 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2016 - fix another bad use of cells, and make sure we catch this bug on Linux

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19976 b5f69653-2bf5-0310-8201-b0dafb8e1405