Fix neglected io.EOF handling

This was probably catched in most cases after commit 9dd92e1, but was
at best misleading as it suggested that the end of input stream signal
from xml.Decoder was intentionally ignored.

Ref mattn/go-xmpp#28

xmpp.go

@@ -833,7 +833,7 @@ type rosterItem struct {
 func nextStart(p *xml.Decoder) (xml.StartElement, error) {
 	for {
 		t, err := p.Token()
-		if err != nil && err != io.EOF || t == nil {
+		if err != nil || t == nil {
 			return xml.StartElement{}, err
 		}
 		switch t := t.(type) {