Fix for mantis 2880: handle -1 red alert departed ship indexes

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9688 387891d4-d844-0410-90c0-e4c51a9137d3
Fix for mantis 1326: add & handle "not present" state for red alert ships

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9689 387891d4-d844-0410-90c0-e4c51a9137d3