avoid retrying a server if its address is unusable

avoid retrying a server if its address is unusable

Previously, if there's no more server to try in force_next,
it attempted to retry the current server similar to a query
timeout case. But, force_next() is called only when the
current server's address is unusable and the query was not
even sent, so recvcount was not incremented, causing an
assertion failure.

We might be able to fix it so the retry doesn't cause a crash,
but it doesn't make sense to retry the server when its address
is known to be unusable. So, simply avoiding the retry would be
the easiest and safest way to prevent the crash.