Fix s/mime certificate deletion bug

Fix s/mime certificate deletion bug

Commit c1bcf4ba exposed a bug in the s/mime encryption code.  It was
errorneously calling unlink on the list of generated cert files to
use.

Prior to that commit, the list had an initial space, which apparently
made the unlink fail.  After that commit, encrypting to a single
certificate would end up deleting the certificate.

Remove the calls to unlink the cert file.  Add some missing cleanup if
the call to openssl fails.

Fixes #1006