Increment of link counter during copy assignment

It is necessary to implement the increment of the reference counter in the assignment operator of the custom smart pointer CustomSharedPtr.

CustomSharedPtr& operator=(const CustomSharedPtr& p) {
    if (p.resource != this->resource) {
        ~CustomSharedPtr();
        this->resource = p.resource;
        this->refCount = p.refCount;
    }
    return *this;
}