Fixed external link
Fixed external link

The BSD Heap Smashing link is dead. 
New link has identical text. Proof: https://web.archive.org/web/20120502090549/http://freeworld.thc.org/root/docs/exploit_writing/BSD-heap-smashing.txt
OverTheWireOrg/OverTheWire-website,OverTheWireOrg/OverTheWire-website,OverTheWireOrg/OverTheWire-website,OverTheWireOrg/OverTheWire-website,OverTheWireOrg/OverTheWire-website