Send termination signals unless the computer is off

If a shutdown has been queued, then the abort message was not set. This
allowed for programs to run for a significantly longer period of time.

src/main/java/dan200/computercraft/core/computer/Computer.java

@@ -285,7 +285,7 @@ public void abort( boolean hard )
     {
         synchronized( this )
         {
-            if( m_state == State.Running )
+            if( m_state != State.Off && m_machine != null )
             {
                 if( hard ) 
                 {