Fix to prevent invalid weapon bitmap references from being created in the lab.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10016 387891d4-d844-0410-90c0-e4c51a9137d3
Assert to trap lua getSignature access to objects with invalid signature.

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10017 387891d4-d844-0410-90c0-e4c51a9137d3