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