Fix bug in JonkerVolgenantSparseAlgorithm. Performance is OK!

It seems like the overhead for the sparse version running a non-sparse
matrix is well acceptable. Hourray! A factor of 2 or 3 for small nRows
(<100) and something that gets close to 1 otherwise.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>