lua: fix tuple cdata collecting

In some cases luajit does not collect cdata objects which were
transformed with ffi.cast as tuple_bless does. In consequence, internal
table with gc callback overflows and then lua crashes. There might be an
internal luajit issue because it fires only for jitted code. But assigning
a gc callback before transformation fixes the problem.

Closes #3751

src/box/lua/tuple.lua

@@ -100,7 +100,7 @@ local tuple_bless = function(tuple)
     -- overflow checked by tuple_bless() in C
     builtin.box_tuple_ref(tuple)
     -- must never fail:
-    return ffi.gc(ffi.cast(const_tuple_ref_t, tuple), tuple_gc)
+    return ffi.cast(const_tuple_ref_t, ffi.gc(tuple, tuple_gc))
 end
 
 local tuple_check = function(tuple, usage)