Move the filling of DRBs to set up for UE context setup/modification
request into the corresponding functions, and use malloc() deliberately
to avoid memory leaks. In fact fill_drb_to_be_setup_from_e1_resp()
allocates memory dynamically (and has to do that), so the easiest is to
do the same for DRBs. The most straight-forward way seems to be to then
dynamically allocate all memory for F1 UE context setup/modification
request messages, and free with the corresponding free functions,
which removes a couple of memory leaks.