Segfault whilst reading message

edit

Hi we've been writting some code to subscribe to the /joint_states topic produced by the husky simulator. However we found that even though all the arrays in /joint_states should be the same length, we are getting an empty array for velocity at random intervals. Which causes a seg fault as there appears to be no way to detect it. (checking if msg->velocity == NULL won't compile). So two questions, is there a way to stop these invalid messages? And if there isn't what is the best way to detect the empty array (gdb says its a boost shared vector).

Thanks, Harry