Fix potential warning in music.c

I saw this included in PR966, but it has nothing to do with the
use-after-free that the pull request is meant to fix. It should
get applied independently of that pull request.

src/music.c

@@ -886,6 +886,8 @@ obj_to_instr(struct obj *obj) {
             ret_instr = ins_melodic_tom;
             break;
     }
+#else
+    nhUse(obj);
 #endif
     return ret_instr;
 }