Make error details conditional on there being details to show.

lib/libvcc/vcc_action.c

@@ -163,6 +163,7 @@ parse_new(struct vcc *tl)
 		VSB_printf(tl->sb, "Object name '%.*s' already used.\n",
 		    PF(tl->t));
 
+		if (sy1->def_b) {
 			VSB_printf(tl->sb, "First usage:\n");
 			AN(sy1->def_b);
 			if (sy1->def_e != NULL)
@@ -171,6 +172,7 @@ parse_new(struct vcc *tl)
 				vcc_ErrWhere(tl, sy1->def_b);
 			VSB_printf(tl->sb, "Redefinition:\n");
 			vcc_ErrWhere(tl, tl->t);
+		}
 		return;
 	}