Fix for Coverity 1107329: Improper use of negative value

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10392 387891d4-d844-0410-90c0-e4c51a9137d3
Fix for Coverity 1147623: Out-of-bounds access

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10393 387891d4-d844-0410-90c0-e4c51a9137d3