Bugfix: socket_t move assignment doesn't initialize ctxptr

Until now, we only assigned the _handle on
`zmq::socket_t::operator=(socket_t&&)`. This manifests when trying to
monitor a socket initialized by that constructor.

To avoid changing the public interface of the socket_t class, we tested
for the specific monitor usecase, since it's the only class accessing
zmq::socke_t::ctxptr.

NOTE: When running the new unit-test without the fix, it might hang on
`zmq_socket_monitor(socket_, NULL, 0)`. We haven't figured out the cause
for that, but we deemed it unimportant (or at least out of scope).

tests/monitor.cpp

@@ -142,4 +142,19 @@ TEST_CASE("monitor init abort", "[monitor]")
     monitor.abort();
     thread.join();
 }
+
+
+TEST_CASE("monitor from move assigned socket", "[monitor]")
+{
+    zmq::context_t ctx;
+    zmq::socket_t sock;
+    sock = std::move([&ctx] {
+        zmq::socket_t sock(ctx, ZMQ_DEALER);
+        return sock;
+    }());
+    zmq::monitor_t monitor1;
+    monitor1.init(sock, "inproc://monitor-client");
+    // On failure, this test might hang indefinitely instead of immediately
+    // failing
+}
 #endif

zmq.hpp

@@ -2103,6 +2103,7 @@ class socket_t : public detail::socket_base
     {
         close();
         std::swap(_handle, rhs._handle);
+        std::swap(ctxptr, rhs.ctxptr);
         return *this;
     }
 #endif
@@ -2121,6 +2122,7 @@ class socket_t : public detail::socket_base
         int rc = zmq_close(_handle);
         ZMQ_ASSERT(rc == 0);
         _handle = ZMQ_NULLPTR;
+        ctxptr = ZMQ_NULLPTR;
     }
 
     void swap(socket_t &other) ZMQ_NOTHROW
@@ -2143,6 +2145,9 @@ class socket_t : public detail::socket_base
     {
         if (_handle == ZMQ_NULLPTR)
             throw error_t();
+        if (ctxptr == ZMQ_NULLPTR)
+            throw error_t();
+
     }
 };