crash if no port is specified in --serve

[svofski]

$ msxnet --serve
Serving workstation 127

Program received signal EXC_BAD_INSTRUCTION, Illegal instruction/operand.
0x0000000100003a2e in SerialPort::waitRx (this=0x7fff5fbff5a8) at serial.cpp:65
65          FD_SET(m_fd, &fdSet);
(gdb) bt
#0  0x0000000100003a2e in SerialPort::waitRx (this=0x7fff5fbff5a8) at serial.cpp:65
#1  0x0000000100008a43 in SpyTransport::Poll (this=0x7fff5fbef448, studentNo=127 '�') at spy.cpp:316
#2  0x0000000100008d25 in Spy::Serve (this=0x7fff5fbff5a8) at spy.cpp:213
#3  0x000000010000207b in main (argc=2, argv=0x7fff5fbffb58) at msxnet.cpp:188