Incorrect sorted pair interactions for stars

As discussed in !776 (merged) (result of #561 (closed)), the star pair tasks do not seem to return the correct results any more (as reported by @aborissov). When running with the --enable-naive-interactions-stars flag or with an older version of the correct number of neighbours is found but not when using the default setup currently in master. This issue affects both the single-node and MPI versions.