multiplexed_socket: Fix memcpy() of nullptr

`memcpy()` has the `__nonnull__` and ASAN doesn't like it even tho the length
of the buffer is 0.  Thus, using a dummy buffer on the stack.

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#0 0x55555a0a1b8a in /usr/include/msgpack/v1/sbuffer.hpp:74
#1 0x55555a1dcfd3 in /usr/include/msgpack/v1/pack.hpp:623
#3 0x55555a11eab2 in /usr/include/msgpack/v1/pack.hpp:1311
#4 0x55555a35c1c5 in /ring-project/daemon/src/jamidht/multiplexed_socket.cpp:676
#5 0x55555a363879 in /ring-project/daemon/src/jamidht/multiplexed_socket.cpp:945
#6 0x55555a35554e in /ring-project/daemon/src/jamidht/multiplexed_socket.cpp:459
#7 0x55555a34e0c0 in /ring-project/daemon/src/jamidht/multiplexed_socket.cpp:247
#8 0x55555a37298f in /ring-project/daemon/src/jamidht/multiplexed_socket.cpp:75
(...)
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Change-Id: Ibc8c8d808c233da1649f556466b24d68decf85e8

src/jamidht/multiplexed_socket.cpp

@@ -658,6 +658,8 @@ MultiplexedSocket::write(const uint16_t& channel,
                          std::size_t len,
                          std::error_code& ec)
 {
+    assert(nullptr != buf);
+
     if (pimpl_->isShutdown_) {
         ec = std::make_error_code(std::errc::broken_pipe);
         return -1;
@@ -942,7 +944,8 @@ ChannelSocket::shutdown()
     stop();
     if (auto ep = pimpl_->endpoint.lock()) {
         std::error_code ec;
-        ep->write(pimpl_->channel, nullptr, 0, ec);
+        const uint8_t dummy = '\0';
+        ep->write(pimpl_->channel, &dummy, 0, ec);
     }
 }