CVE-2017-13690/IKEv2: Fix some bounds checks.

Use a pointer of the correct type in ND_TCHECK(), or use ND_TCHECK2()
and provide the correct length.

While we're at it, remove the blank line between some checks and the
UNALIGNED_MEMCPY()s they protect.

Also, note the places where we print the entire payload.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).
numstr(int x)
{
	static char buf[20];
	snprintf(buf, sizeof(buf), "#%d", x);
	return buf;
}

numstr(int x)
{
	static char buf[20];
	snprintf(buf, sizeof(buf), "#%d", x);
	return buf;
}
