You should only use code published in an official release of OpenZeppelin Contracts, the latest release is 2.3. When importing via GitHub on Remix you can specify the release tag, (otherwise you will get the latest code in the master branch).

The imports using version 2.3.0 of OpenZeppelin Contracts would be:

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.3.0/contracts/token/ERC721/ERC721Full.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v2.3.0/contracts/math/SafeMath.sol";

Raised as an issue on StackOverflow:
https://stackoverflow.com/questions/57540884/warning-regarding-extcodehash-in-remix-for-openzeppelin-contracts/57551294#57551294

Disclosure: I am the Community Manager at OpenZeppelin