Empty mission list causes MACE crash at startup

[patnolan33]

When starting MACE and connecting to a vehicle with an empty mission, MACE will crash and output that it "Cannot initialize queue of 0" (see

OpenMACE/src/data_generic_command_item/mission_items/mission_list.cpp

Line 43 in 39b30b7

std::cout << "Cannot initialize queue of 0" << std::endl;

).

This has been fixed before, but the fix needs to be pulled into OpenMACE.