failing debug assertion in `compute_ts_equal_cost_ranges`

[jeeeesper]

I stumbled upon a weird behaviour in lib_tsalign version 3.0.0. To reproduce, run the following:

//        [-- 10 --][-- 11 ---]1
let r = b"AAAAAAAAAACCCCCCCCCCCC";
let q = b"AAAAAAAAAATTTTTTTTTTTC";

let mut tsa = Aligner::<DnaAlphabetOrN>::default();

let mut costs = TemplateSwitchConfig::default();
costs.base_cost.qrr = 4u64.into();
tsa.set_costs(costs.clone());
let _tsa_res = tsa.align("ref", r, "qry", q, None, None, None, false);

This crashes during TS equal cost range extension from a debug assertion:

thread '::tests::compute_cost_shenanigans' (68464) panicked at <...>/lib_tsalign-3.0.0/src/a_star_aligner/alignment_result.rs:445:17:
assertion `left == right` failed
  left: U64Cost(18446744073709551615)
 right: U64Cost(4)
stack backtrace:
	...
   4: lib_tsalign::a_star_aligner::alignment_result::AlignmentResult<lib_tsalign::a_star_aligner::template_switch_distance::alignment_type::AlignmentType,Cost>::compute_ts_equal_cost_ranges
             at <...>/lib_tsalign-3.0.0/src/a_star_aligner/alignment_result.rs:445:17
   5: lib_tsalign::a_star_aligner::template_switch_distance_a_star_align
             at <...>/lib_tsalign-3.0.0/src/a_star_aligner.rs:252:12
	...

The failing assertion reads
debug_assert_eq!(current_cost, (statistics.cost.round().raw() as u64).into());