OpenSSL EVP: fix threaded use of structs

Make sure we don't clear or reset static structs after first init so
that they work fine even when used from multiple threads. Init the
structs in the global init.

Help and assistance by: John Engstrom

Fixes #229 (again)

src/crypto.h

@@ -113,4 +113,6 @@ int _libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
                               const char *privatekey,
                               const char *passphrase);
 
+void _libssh2_init_aes_ctr(void);
+
 #endif