fix memory leak in /cacert operation on clientside

[McRoot12]

there is a memory leak on the clientside

openssl manpage (https://www.openssl.org/docs/man1.0.2/crypto/X509_STORE_CTX_init.html):

X509_STORE_CTX_cleanup() internally cleans up an X509_STORE_CTX structure. The context can then be reused with an new call to X509_STORE_CTX_init().