setting to 0 is better because -1 triggers an Int3() on the first swarm entry

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9890 387891d4-d844-0410-90c0-e4c51a9137d3
make collider_pair an actual class since its constructor is called by hash_map

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9891 387891d4-d844-0410-90c0-e4c51a9137d3