reject old ZMTP connections if auth enabled

auth mechanisms were only enabled when ZMTP handshake
is latest version, meaning that connections from old sockets
would skip authentication altogether

src/stream_engine.cpp

@@ -534,6 +534,12 @@ bool zmq::stream_engine_t::handshake ()
     //  Is the peer using ZMTP/1.0 with no revision number?
     //  If so, we send and receive rest of identity message
     if (greeting_recv [0] != 0xff || !(greeting_recv [9] & 0x01)) {
+        if (session->zap_connect () == 0) {
+           // reject ZMTP 1.0 connections if ZAP is enabled
+           error (protocol_error);
+           return false;
+        }
+
         encoder = new (std::nothrow) v1_encoder_t (out_batch_size);
         alloc_assert (encoder);
 
@@ -575,6 +581,12 @@ bool zmq::stream_engine_t::handshake ()
     }
     else
     if (greeting_recv [revision_pos] == ZMTP_1_0) {
+        if (session->zap_connect () == 0) {
+           // reject ZMTP 1.0 connections if ZAP is enabled
+           error (protocol_error);
+           return false;
+        }
+
         encoder = new (std::nothrow) v1_encoder_t (
             out_batch_size);
         alloc_assert (encoder);
@@ -585,6 +597,12 @@ bool zmq::stream_engine_t::handshake ()
     }
     else
     if (greeting_recv [revision_pos] == ZMTP_2_0) {
+        if (session->zap_connect () == 0) {
+           // reject ZMTP 2.0 connections if ZAP is enabled
+           error (protocol_error);
+           return false;
+        }
+
         encoder = new (std::nothrow) v2_encoder_t (out_batch_size);
         alloc_assert (encoder);