Possible solution to ticket #55

As reported in ticket #55 SGSN can crash due to double free-ing. You can replace 'can' by 'will' in that last phrase. I had a sift through the code and tried to solve this by removing the free in gprs_ns.c. Whenever the calling function created the msgb-struct, I have made the function free it after its use. If the function got the msgb from a calling function, there will not be a free (hoping that will be done on the higher level). HTH/F