databases/mysql-connector-c: Fix build with Clang 6

sql-common/client_authentication.cc:86:56: error: comparison between pointer and integer ('char *' and 'int')

mysql->options.extension->server_public_key_path != '\0')
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p479076_s338486/logs/errors/mysql-connector-c-6.1.11.log

PR: 230959
Approved by: bofh (maintainer timeout, 2 weeks)