Fix a few missed "if (!ptr)" cleanups

And a scalar !x --> x==0 test
Reviewed-by: Kurt Roeckx <kurt@openssl.org>

ssl/ssl_sess.c

@@ -466,7 +466,7 @@ int ssl_get_new_session(SSL *s, int session)
          * Don't allow the callback to set the session length to zero. nor
          * set it higher than it was.
          */
-        if (!tmp || (tmp > ss->session_id_length)) {
+        if (tmp == 0 || tmp > ss->session_id_length) {
             /* The callback set an illegal length */
             SSLerr(SSL_F_SSL_GET_NEW_SESSION,
                    SSL_R_SSL_SESSION_ID_HAS_BAD_LENGTH);