gb: Fix gprs_nsvc_delete() to free ctr group

This fixes a SEGV error that happens the next time the statistics
are updated.

Addresses:
 Program terminated with signal 11, Segmentation fault.
 #0  0xb7711fa5 in rate_ctr_group_intv (grp=<optimized out>) at
 rate_ctr.c:107
 107             for (i = 0; i < grp->desc->num_ctr; i++) {
 #0  0xb7711fa5 in rate_ctr_group_intv (grp=<optimized out>) at
     rate_ctr.c:107
 #1  rate_ctr_timer_cb (data=0x0) at rate_ctr.c:129
 #2  0xb770ec59 in osmo_timers_update () at timer.c:243
 #3  0xb770ef7a in osmo_select_main (polling=0) at select.c:133
 #4  0x08049987 in main (argc=3, argv=0xbfba8084) at
     gb_proxy_main.c:306

Sponsored-by: On-Waves ehf
3 files changed