Assigning `@post` variables should not affect the original array

Fixes https://github.com/rubinius/rubinius/issues/3037.

Before:
```ruby
arr = [1, 2, 3, 4, 5]
*a, b, c, d = *arr
arr # => [1, 2]
```

After:
```ruby
arr = [1, 2, 3, 4, 5]
*a, b, c, d = *arr
arr # => [1, 2, 3, 4, 5]
```

Oddly enough this only happened when the splat operator is at the beginning (`a, *b, c` worked just fine)