Make sure entries in the Debris[] array get cleared properly upon deletion


git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10007 387891d4-d844-0410-90c0-e4c51a9137d3
Adds a check to the mn.Debris table in lua to check for invalid debris pieces


git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10008 387891d4-d844-0410-90c0-e4c51a9137d3