[SGSN] Fix segfault ar PDP CTX DEACT time

We need to call rate_ctr_group_free() on the per-PDPctx-counters
diff --git a/openbsc/src/gprs/gprs_sgsn.c b/openbsc/src/gprs/gprs_sgsn.c
index 9a76cee..ae18ea0 100644
--- a/openbsc/src/gprs/gprs_sgsn.c
+++ b/openbsc/src/gprs/gprs_sgsn.c
@@ -205,6 +205,7 @@
 
 void sgsn_pdp_ctx_free(struct sgsn_pdp_ctx *pdp)
 {
+	rate_ctr_group_free(pdp->ctrg);
 	llist_del(&pdp->list);
 	llist_del(&pdp->g_list);
 	talloc_free(pdp);