remove gcc warning: [-Wunused-but-set-variable]

(ret_val earmarked for future use)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10088 387891d4-d844-0410-90c0-e4c51a9137d3
Coverity 1129270: Negative array index read

weapon_create can return -1
(& oops, r10087 was 1129271, not 1129270)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10089 387891d4-d844-0410-90c0-e4c51a9137d3
Coverity 1129274: Negative array index read

weapon_create can return -1

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10090 387891d4-d844-0410-90c0-e4c51a9137d3