wtclient: make addr iterator panic safe

Ensure that calling Next twice in a row without first calling Reset is
safe when the iterator is at the end of its list. Also alter the
towerListIterator to call Reset after hitting an error on Next.

watchtower/wtclient/addr_iterator.go

@@ -162,6 +162,10 @@ func (a *addressIterator) next(lock bool) (net.Addr, error) {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
+	if a.currentTopAddr == nil {
+		return nil, ErrAddressesExhausted
+	}
+
 	// Set the next candidate to the subsequent element.
 	a.currentTopAddr = a.currentTopAddr.Next()