Some users reported that MPD crashes when using a new CURL version
with the threaded DNS resolver enabled.  It seems that
curl_multi_fdset() returns no file descriptor when the DNS resolver
runs in another thread, so MPD does not have any event to wait for.

On the CURL mailing list, somebody suggested to sleep for a fixed
amount of time.  This is not an elegant solution, because daemons
should never have to sleep without waiting for an event.  I hope the
CURL developers will review the API and remove the threaded DNS
resolver.

Meanwhile, I'm removing the assertion in question, to allow those
unfortunate users running the latest CURL version to continue using
MPD.