CVE-2017-5485/Fix lookup_nsap() to match what isonsap_string() expects.

CVE-2017-5485/Fix lookup_nsap() to match what isonsap_string() expects.

Change cddcb5632decb3fc631407b2ce0e526d17bb8ff9 changed isonsap_string()
to take, as arguments, a pointer to the first octet of an NSAP and the
length of the NSAP, rather than a pointer to a string of octets the
first octet of which is the NSAP length and the subsequent octets are
the octets of the NSAP.

However, lookup_nsap() was not changed in a similar fashion, and
isonsap_string() handed it a pointer to the first octet of the NSAP,
which lookup_nsap() treated as the NSAP length.

This should fix GitHub issue #563.