Some more netcode fixes for n-segment shields.

git-svn-id: svn://svn.icculus.org/fs2open/branches/zookeeper@9952 387891d4-d844-0410-90c0-e4c51a9137d3
Guard against possible out-of-bounds shield quadrant indexing.

git-svn-id: svn://svn.icculus.org/fs2open/branches/zookeeper@9953 387891d4-d844-0410-90c0-e4c51a9137d3