Fix infinite loop in ApproximateSetCover at bucket 0 due to unsigned underflow

[alanSquirrelyz]

cur_bkt is size_t, so when cur_bkt == 0, cur_bkt - 1 wraps to SIZE_MAX. This makes pow(1.0 + epsilon, SIZE_MAX) evaluate to infinity, and size_t(infinity) is UB that yields a huge low_threshold. No vertex can satisfy numWon >= low_threshold, causing an infinite loop at bucket 0 for any graph with degree-1 vertices.

----------------------------------------ORIGINAL-----------------------------------------

    const size_t low_threshold =
        std::max((size_t)ceil(pow(1.0 + sc::epsilon, cur_bkt - 1)), (size_t)1);

------------------------------------------FIXED--------------------------------------------

    const size_t low_threshold =
        (cur_bkt == 0) ? (size_t)1
                       : std::max((size_t)ceil(pow(1.0 + sc::epsilon, cur_bkt - 1)), (size_t)1);