* include/parallel/multiway_merge.h
        (multiway_merge_loser_tree):
        Leave checks to callers, add precondition instead.
        (multiway_merge_loser_tree_unguarded): Likewise.
        (multiway_merge_loser_tree_sentinel): Likewise.
        (sequential_multiway_merge): Added checks for total length 0.
        (parallel_multiway_merge): Skip empty sequences.
        (multiway_merge, all variants):
        Remove temporary variable, return directly.
        (stable_multiway_merge, all variants): Likewise.
        (multiway_merge_sentinels, all variants):  Likewise.
        (stable_multiway_merge_sentinels, all variants): Likewise.
        * include/parallel/multiseq_selection.h
        (multiseq_partition): More detailed assertions.

From-SVN: r134580