Adds a check for force shutdown timeout system property.

src/net/java/sip/communicator/impl/shutdowntimeout/ShutdownTimeout.java

@@ -93,8 +93,13 @@ public void run()
                             if(shutdownCustomValue != null
                                 && shutdownCustomValue.length() > 0)
                             {
-                                shutDownTimeout =
+                                long custom =
                                     Long.valueOf(shutdownCustomValue);
+
+                                // make sure custom is not 0, or it will
+                                // wait forever
+                                if(custom > 0)
+                                    shutDownTimeout = custom;
                             }
                         }
                         catch(Throwable t){}