smartlist_string_num_isin's buffer is too short

smartlist_string_num_isin converts an int to a 15-character or shorter string. Its 16-byte buffer is not long enough to handle all 64-bit ints (-2^63^ is 20 characters long, so the buffer would need to be at least 21 bytes long).

We currently only use this function for (16-bit) TCP port numbers, so this bug doesn't actually hurt anything, but we shouldn't leave an even slightly incorrect function lying around to bite us later.