fix Mantis #2955: crash when trying to join multiplayer server, caused by trying to memcpy an integer into a ushort
(also add some sanity asserts)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10117 387891d4-d844-0410-90c0-e4c51a9137d3