[Hw4] Replace ill-formed assignment operator

[YangWithU]

The copy-assignment operator seems redudunt and ill-formed.
What if error was thrown during this->ref_counter = rhs.ref_counter; and this->base_ptr = rhs.base_ptr; ?
Find a better solution
Article as a guidance

template <typename T>
SharedPtr<T>& SharedPtr<T>::operator=(SharedPtr& rhs) {
    if(this != &rhs) {
        (*rhs.ref_counter) += 1;
        this->ref_counter = rhs.ref_counter;
        this->base_ptr = rhs.base_ptr;
    }
    return *this;
}