Function btck_chain_get_by_height 

pub unsafe extern "C" fn btck_chain_get_by_height(
    chain: *const btck_Chain,
    block_height: c_int,
) -> *const btck_BlockTreeEntry

@brief Retrieve a block tree entry by its height in the currently active chain. Once retrieved there is no guarantee that it remains in the active chain.

@param[in] chain Non-null. @param[in] block_height Height in the chain of the to be retrieved block tree entry. @return The block tree entry at a certain height in the currently active chain, or null if the height is out of bounds.