Coverity 1129273: Negative array index read

weapon_create can return -1

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10085 387891d4-d844-0410-90c0-e4c51a9137d3
Coverity 1129272: Negative array index read

weapon_create can return -1

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10086 387891d4-d844-0410-90c0-e4c51a9137d3