Add and use OPENSSL_zalloc

There are many places (nearly 50) where we malloc and then memset.
Add an OPENSSL_zalloc routine to encapsulate that.
(Missed one conversion; thanks Richard)
Also fixes GH328

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_sess.c

@@ -193,12 +193,11 @@ SSL_SESSION *SSL_SESSION_new(void)
 {
     SSL_SESSION *ss;
 
-    ss = OPENSSL_malloc(sizeof(*ss));
+    ss = OPENSSL_zalloc(sizeof(*ss));
     if (ss == NULL) {
         SSLerr(SSL_F_SSL_SESSION_NEW, ERR_R_MALLOC_FAILURE);
         return (0);
     }
-    memset(ss, 0, sizeof(*ss));
 
     ss->verify_result = 1;      /* avoid 0 (= X509_V_OK) just in case */
     ss->references = 1;