Use FREE_OBJ for VEV

This was actually done with a Coccinelle patch after finding plain
free() calls out of tree:

    $ cat >free_obj.cocci <<EOF
    @@
    expression obj, objp, magic;
    @@

    (
    TAKE_OBJ_NOTNULL(obj, objp, magic);
    |
    CAST_OBJ_NOTNULL(obj, objp, magic);
    |
    CHECK_OBJ_NOTNULL(obj, magic);
    |
    CHECK_OBJ_ORNULL(obj, magic);
    |
    CHECK_OBJ(obj, magic);
    )
    ...
    - free(obj);
    + FREE_OBJ(obj);
    $ spatch --dir . --in-place --sp-file free_obj.cocci

Asking again, could we consider keeping Coccinelle patches around?

lib/libvarnish/vev.c

@@ -222,9 +222,8 @@ VEV_Destroy(struct vev_root **evbp)
 	assert(evb->thread == pthread_self());
 	free(evb->pfd);
 	free(evb->pev);
-	/* destroy evb->binheap */
-	evb->magic = 0;
-	free(evb);
+	/* XXX: destroy evb->binheap */
+	FREE_OBJ(evb);
 }
 
 /*--------------------------------------------------------------------*/