Possible data corruption when assigning variant to itself

@artemp is already working on this.

Since Jan 11, 2018.

Hi, Thanks to you variant implementation!
But i see, that there are issue in member
```
VARIANT_INLINE variant& operator=(variant const& other)
{
copy_assign(other);
return *this;
}
```

Now there are no check for `&other ==this` and `copy_assign` can destroy variant member data's before actual copy to itself
```
VARIANT_INLINE void copy_assign(variant const& rhs)
{
helper_type::destroy(type_index, &data);
type_index = detail::invalid_value;
helper_type::copy(rhs.type_index, &rhs.data, &data);
type_index = rhs.type_index;
}
```

So, in result `helper_type::copy(rhs.type_index, &rhs.data, &data);` can operate with invalid data

Best Regards