Coverity ID 1093586 Negative array index read

Shouldn't be able to occur since when artillery_objnum is set to -1,
artillery_lock_time is also set to 0.0f. BUT, protect the array
access anyway in case the surrounding code changes in future.

git-svn-id: svn://svn.icculus.org/fs2open@10794 387891d4-d844-0410-90c0-e4c51a9137d3