Fix use-after-free in memtx_tuple_delete()

Struct of type tuple_format is being passed as an argument to
tuple_format_unref() where it might be freed. On such occasion any
further references to format fields should not take place.

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>

Closes #4658

(cherry picked from commit c08b94ed)