Fix possible NPE in roster push listener
Daniele Ricci reporting the following NPE using Smack 4.1.9
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.indexOf(int)' on a null object reference
at org.jxmpp.util.XmppStringUtils.parseBareJid(XmppStringUtils.java:124)
at org.jivesoftware.smack.roster.Roster$RosterPushListener.handleIQRequest(Roster.java:1416)
at org.jivesoftware.smack.AbstractXMPPConnection$2.run(AbstractXMPPConnection.java:1061)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
at java.lang.Thread.run(Thread.java:818)
This is possibly caused by a service sending roster pushes for unbound
connections, i.e. where getUsers() returns 'null'. We now log such
situations instead throwing an NPE.
