Extract lowest bits

A device packs several fields into one non-negative integer, and the lowest k bits hold a rolling counter. Given the packed value n (0 <= n < 2^31) and a width k (0 <= k <= 30), return the counter: the integer formed by the lowest k bits of n. When k is 0 the counter is empty, so return 0. For example, n = 173 (binary 10101101) with k = 4 returns 13 (binary 1101).