[VectorUtils] Do not try to add indices matching tombstone/empty values.

Keys matching the tombstone/empty special values cannot be inserted in a
DenseMap. Under some circumstances, LV tries to add members to an
interleave group that match the special values. Skip adding such
members. This is unlikely to have any impact in practice, because
interleave groups with such indices are very likely to not be
vectorized, due to gaps.

This issue has been surfaced by fuzzing, see
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11638