Fixed one warning message at shutdown after checkpoint load and kept the current multicast group after checkpoint load.

[hchen99]

1. When an external app such as the sim control panel is configured as following in the input file, the corresponding destructor gets called at the shutdown. In turn, the MM will try to clean the memory for the vars related to the external app even those vars were already taken care of due to the shutdown. Thus there was a warning message.
   simControlPanel = trick.SimControlPanel()
trick.add_external_application(simControlPanel)
   However, if having trick.sim_control_panel_set_enabled(True) in the input file, there was no warning message.

Deleted the unnecessary command c str pointer, thus removed the need to free its memory allocation as before.

2. Made sure that the multicast group is not initialized before calling a function to initialize it in VariableServerListenThread.cpp. Without the check, trick-sniffer wouldn't find the sim anymore after loading a checkpoint as the multicast group got initialized again even it was already initialized.

[coveralls]

coverage: 55.83% (-0.06%) from 55.892%
when pulling c1f4af1 on fix-two-warning-messages-related-to-checkpoint-load
into eed8707 on master.