0 is a valid socket description, so should be accepted after we create a
socket. In IDF, though, it's just a theoretical issue, as customers have
to configure:
* FD_SETSIZE = CONFIG_LWIP_MAX_SOCKETS (from makefiles)
* CONFIG_VFS_SUPPORT_IO = n (from menuconfig)
(this configuration is currently broken, but if we adjust the IDF here
and there we could reproduce the issue)