when parsing omemo messages ensure we only find one element

src/main/java/eu/siacs/conversations/crypto/axolotl/XmppAxolotlMessage.java

@@ -135,7 +135,7 @@ public class XmppAxolotlMessage {
 					break;
 			}
 		}
-		Element payloadElement = axolotlMessage.findChild(PAYLOAD); //TODO make sure we only have _one_ paypload
+		final Element payloadElement = axolotlMessage.findChildEnsureSingle(PAYLOAD, AxolotlService.PEP_PREFIX);
 		if (payloadElement != null) {
 			ciphertext = Base64.decode(payloadElement.getContent().trim(), Base64.DEFAULT);
 		}

src/main/java/eu/siacs/conversations/parser/MessageParser.java

@@ -338,7 +338,7 @@ public class MessageParser extends AbstractParser implements OnMessagePacketRece
         final URL xP1S3url = xP1S3 == null ? null : P1S3UrlStreamHandler.of(xP1S3);
         final String oobUrl = oob != null ? oob.findChildContent("url") : null;
         final String replacementId = replaceElement == null ? null : replaceElement.getAttribute("id");
-        final Element axolotlEncrypted = packet.findChild(XmppAxolotlMessage.CONTAINERTAG, AxolotlService.PEP_PREFIX); //TODO make sure we only have _one_ axolotl element!
+        final Element axolotlEncrypted = packet.findChildEnsureSingle(XmppAxolotlMessage.CONTAINERTAG, AxolotlService.PEP_PREFIX);
         int status;
         final Jid counterpart;
         final Jid to = packet.getTo();

src/main/java/eu/siacs/conversations/xml/Element.java

@@ -80,6 +80,19 @@ public class Element {
 		return null;
 	}
 
+	public Element findChildEnsureSingle(String name, String xmlns) {
+		final List<Element> results = new ArrayList<>();
+		for (Element child : this.children) {
+			if (name.equals(child.getName()) && xmlns.equals(child.getAttribute("xmlns"))) {
+				results.add(child);
+			}
+		}
+		if (results.size() == 1) {
+			return results.get(0);
+		}
+		return null;
+	}
+
 	public String findChildContent(String name, String xmlns) {
 		Element element = findChild(name,xmlns);
 		return element == null ? null : element.getContent();