socket_client.cpp: prevent buffer overflow in creation of unix socket

socket_client.cpp: prevent buffer overflow in creation of unix socket

Path size for UNIX domain sockets is fixed at 108, however, the
method ``.unix_connect()`` of the socket client class accepts
STL strings of any length unchecked. Thus it’s trivial to provoke
a segfault:

    libt2n::socket_client_connection sc(std::string(42 * 42, '!'));

and ... bang!

A check of the client-supplied path value against the buffer size
of ``sockaddr_un.sun_path[]`` is added to prevent the issue.