Fix compile error error about array initialization

[ivangalkin]

DialDiscovery.cpp:317:19: error: variable-sized object may not be initialized
    char send_buf[strlen((char*)ssdp_msearch) + INET_ADDRSTRLEN + 256] = {0,};
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix by means of replacing run-time length calculation with a compile-time calculation.