client: Fix 32-bit overflow when computing pcapng EPB timestamp
tv_sec * 1000 * 1000 was evaluated in int arithmetic. Where time_t / tv_sec is 32-bit, this overflows for any tv_sec > ~2147, corrupting the 64-bit timestamp_usec well before the year 2038. Cast tv_sec to uint64_t before the multiplication so the whole expression is computed in 64 bits.
