Sets the key method data of an EC_KEY object, if none has yet been set.
\param key EC_KEY object
\param data opaque data to install.
\param dup_func a function that duplicates |data|.
\param free_func a function that frees |data|.
\param clear_free_func a function that wipes and frees |data|.
\return the previously set data pointer, or null if |data| was inserted.
Sets the key method data of an EC_KEY object, if none has yet been set. \param key EC_KEY object \param data opaque data to install. \param dup_func a function that duplicates |data|. \param free_func a function that frees |data|. \param clear_free_func a function that wipes and frees |data|. \return the previously set data pointer, or null if |data| was inserted.