BUG/MEDIUM: ssl: Clear OpenSSL error stack after trying to parse OCSP file

BUG/MEDIUM: ssl: Clear OpenSSL error stack after trying to parse OCSP file

Invalid OCSP file (for example empty one that can be used to enable
OCSP response to be set dynamically later) causes errors that are
placed on OpenSSL error stack. Those errors are not cleared so
anything that checks this stack later will fail.

Following configuration:
  bind :443 ssl crt crt1.pem crt crt2.pem

With following files:
  crt1.pem
  crt1.pem.ocsp - empty one
  crt2.pem.rsa
  crt2.pem.ecdsa

Will fail to load.

This patch should be backported to 1.7.