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.
That happens when one removes the "&& ..." part in the assert, thanks for pointing this out.

Personally I would not output an error message at all but only stick to the assert, but as far as I've seen SFML uses error messages a lot, so I added one.

Actually an exception would even fit better here, however SFML does not use them yet. (Exception because the developer can't do the check himself, because the descriptors are internal)

I'm fine with an error message only and change it at a later point when error checking is reconsidered in general.