Check socket descriptor limit. #153

When calling select(), there's an upper limit for the socket descriptor
which is defined as FD_SETSIZE. When the socket descriptor is higher
than FD_SETSIZE, a call to select() will not work as expected, at least
for the proper sockets.

This patch adds an error message and an assert as well.
This assertion will never fail, since the `char*` pointer is always valid (pointing to a string literal) and thus converted to `true`.

In general, SFML usually does not assert if it outputs error messages at the same time, I don't know whether we want to change this policy or rather use one or the other...