CVE-2017-13008/IEEE 802.11: Fix TIM bitmap copy to copy from p + offset.

offset has already been advanced to point to the bitmap; we shouldn't
add the amount to advance again.

This fixes a buffer over-read discovered by Brian 'geeknik' Carpenter.

Add a test using the capture file supplied by the reporter(s).

While we're at it, remove some redundant tests - we've already checked,
before the case statement, whether we have captured the entire
information element and whether the entire information element is
present in the on-the-wire packet; in the cases for particular IEs, we
only need to make sure we don't go past the end of the IE.
handle_atim(void)
{
	/* the frame body for ATIM is null. */
	return 1;
}

handle_atim(void)
{
	/* the frame body for ATIM is null. */
	return 1;
}
