Fix building on gcc 7

[Thomas Deutschmann]

dev-db/percona-server-5.6.37.82.2 fails to build with gcc-7.2.0:

work/mysql/sql-common/client_authentication.cc:87:59: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
       mysql->options.extension->server_public_key_path != '\0')
                                                           ^~~~

MySQL upstream fixed this in mysql-8 long time ago via https://github.com/mysql/mysql-server/commit/db1bde79b1b4c0bd6864f80de4e2c233aef5d025#diff-763b8fc269ea583b440d0847c7c1f05d