[INTERNAL] prevent possible NPE while decorating the chat

java.lang.NullPointerException
at
de.fu_berlin.inf.dpp.ui.widgets.chat.items.ChatLine.isValidURL(ChatLine.java:170)

The Javadoc does not state that it will return null. I just looked into
the source code of the JVM Oracle distribution and found not anything
that will assign null to the host string.

But as it can happen just handle that case.

Change-Id: I3ace00e9cb2f33b4a9f35642da402801880d2f52
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1387
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>