Does not run correctly on Windows

[RC1844]

There is a bug in Windows, the problem is in 192fac7 commit introduced

libmodbus/src/modbus.c

Line 371 in dd45f19

if (ctx->s < 0) {

change the commit did not deal with the case of running under Windows

[devotip]

on windows when modbus rtu is opened looking at ctx->s is incorrect because the serial handle is in
modbus_rtu_t *ctx_rtu = ctx->backend_data;
ctx_rtu->w_ser.fd

To stay on the clean side it is probably appropriate to add a backend method "checkconnected" to differentiate the handling

[devotip]

Meanwhile the pull request gets accepted you can try this https://github.com/devotip/libmodbus

[RC1844]

Oh,good.Expect pull request