net/udt: fix build on 14.1-RELEASE

core.cpp:2465:19: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
 2465 |    char cookiestr[(AF_INET == addr->sa_family ? INET_ADDRSTRLEN : INET6_ADDRSTRLEN) +
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2466 |       sizeof(clientport) + 64];
      |       ~~~~~~~~~~~~~~~~~~~~~~~
core.cpp:2465:31: note: function parameter 'addr' with unknown value cannot be used in a constant expression
 2465 |    char cookiestr[(AF_INET == addr->sa_family ? INET_ADDRSTRLEN : INET6_ADDRSTRLEN) +
      |                               ^
core.cpp:2452:28: note: declared here
 2452 | int CUDT::listen(sockaddr* addr, CPacket& packet)
      |                            ^
1 error generated.