QSslCertificate (OpenSSL) use the correct *_free function

When releasing a STACK_OF(GENERAL_NAME). Actually, GENERAL_NAME_free is
a special function, not the same as OPENSSL_sk_free.

Task-number: QTBUG-57679
Change-Id: I3ed300bb95e8be35bd9cd06b6dbc6e59c7c6a4ee
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>