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Messages in ROS are compiled into compatible C++ structures. Here joint_state.name is a std::vector<std::string> and joint_state.position is a std::vector<double>. When adding elements to these structures, you need to use the standard methods of adding elements to a C++ vector, such as
joint_state.position.push_back(0.0);

or by first resizing the joint_state.position vector:
joint_state.position.resize(7); // and then index as you do above
joint_state.position[0] = 0.0;

I happen to have a Github Gist that commands Baxter's arms with C++, quite similar to your code (sans the coredump :)

https://gist.github.com/rethink-imcmahon/237e6be4d29d812577be

One potential "gotcha" here is that you will need to command Baxter's arms at a rate greater than 5Hz. Otherwise, they will revert back into position mode and hold their place. This is a safety feature.

And for a few helpful links -

• ROS Message inner workings: http://wiki.ros.org/msg
• Reference for C++ vectors: http://www.cplusplus.com/reference/vector/vector/
• Baxter's Arm Control Mode Details: http://sdk.rethinkrobotics.com/wiki/Arm_Control_Modes