Check error stream is not null before notifying error

[ramkumarr]

Fixes #955

We are getting this exception frequently, when the network to the server is poor (ie., there are a lot of dropped packets at the server, for example):

java.lang.NullPointerException: null
	at net.schmizz.sshj.connection.channel.direct.SessionChannel.notifyError(SessionChannel.java:228) ~[sshj-0.31.0.jar:0.31.0]
	at net.schmizz.sshj.common.ErrorNotifiable$Util.alertAll(ErrorNotifiable.java:35) ~[sshj-0.31.0.jar:0.31.0]
	at net.schmizz.sshj.connection.ConnectionImpl.notifyError(ConnectionImpl.java:261) ~[sshj-0.31.0.jar:0.31.0]
	at net.schmizz.sshj.transport.TransportImpl.die(TransportImpl.java:620) ~[sshj-0.31.0.jar:0.31.0]
	at net.schmizz.sshj.transport.Reader.run(Reader.java:66) ~[sshj-0.31.0.jar:0.31.0]

I can't really see how the err variable in SessionChannel can be null, but apparently sometimes it is.
This kind of check is already done in AbstractChannel.notifyError (although only for the out stream), so I'm doing it in SessionChannel as well.