Having the invalid value of descriptor here might be considered weird, but it happens nevertheless, when you're DoSing the websocket endpoint using the 3G connection. It makes sense to return the generic error here instead of trying to read() from it.
See https://en.cppreference.com/w/cpp/header/cerrno for additional details. Some of used constants are defined in this header.
Inclusion is necessary to avoid these errors:
```
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:189:30: error: use of undeclared identifier 'EAFNOSUPPORT'
default: errno = EAFNOSUPPORT; return 0;
^
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:191:17: error: use of undeclared identifier 'ENOSPC'
errno = ENOSPC;
^
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:175:25: warning: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long long') to 'int' [-Wshorten-64-to-32]
j = strspn(buf + i, ":0");
~ ^~~~~~~~~~~~~~~~~~~~~
/home/user/IXWebSocket/ixwebsocket/IXNetSystem.cpp:234:21: error: use of undeclared identifier 'EAFNOSUPPORT'
errno = EAFNOSUPPORT;
^
2 warnings and 3 errors generated.
```