page_zip_fields_decode(): Do not dereference index=NULL.
Instead, return NULL early. The only caller does not care
about the values of output parameters in that case.

This bug was introduced in MySQL 5.7.6 by
mysql/mysql-server@9eae0edb7a8e4004328e61157f5f3b39cebe1b2b
and in MariaDB 10.2.2 by
commit 2e814d47.

Thanks to my son for pointing this out after investigating
the output of a static analysis tool.