segtree: Use expr_clone in get_set_interval_*()

segtree: Use expr_clone in get_set_interval_*()

Both functions perform interval set lookups with either start and end or
only start values as input. Interestingly, in practice they either see
values which are not contained or which match an existing range exactly.

Make use of the above and just return a clone of the matching entry
instead of creating a new one based on input data.

Signed-off-by: Phil Sutter <phil@nwl.cc>