src: fix memory leak introduced in 34febfbf

Fix leaking the BIO in the error path.  Introduced in commit 34febfbf
("crypto: fix handling of root_cert_store").

PR-URL: https://github.com/nodejs/node/pull/9604
Refs: https://github.com/nodejs/node/pull/9409


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rod Vagg <rod@vagg.org>