Merge pull request #3705 from somdoron/ws_mechanism

problem: zero-sized messages cause assertion when glibc assertion are on

src/curve_mechanism_base.cpp

@@ -70,6 +70,7 @@ int zmq::curve_mechanism_base_t::encode (msg_t *msg_)
     message_plaintext[crypto_box_ZEROBYTES] = flags;
     // this is copying the data from insecure memory, so there is no point in
     // using secure_allocator_t for message_plaintext
+    if (msg_->size () > 0)
         memcpy (&message_plaintext[crypto_box_ZEROBYTES + 1], msg_->data (),
                 msg_->size ());
 
@@ -160,6 +161,7 @@ int zmq::curve_mechanism_base_t::decode (msg_t *msg_)
 
         // this is copying the data to insecure memory, so there is no point in
         // using secure_allocator_t for message_plaintext
+        if (msg_->size () > 0)
             memcpy (msg_->data (), &message_plaintext[crypto_box_ZEROBYTES + 1],
                     msg_->size ());
     } else {