Forward GGSN originated DEACT PDP CTX REQ

This commit fixes TTCN3 sgsn test TC_attach_pdp_act_user_deact_mt.

Change-Id: I204209c017aac8a8402cbb8d0a0200540abcc954
diff --git a/src/gprs/sgsn_libgtp.c b/src/gprs/sgsn_libgtp.c
index 88cea5d..659392e 100644
--- a/src/gprs/sgsn_libgtp.c
+++ b/src/gprs/sgsn_libgtp.c
@@ -639,8 +639,15 @@
 /* Called whenever a PDP context is deleted for any reason */
 static int cb_delete_context(struct pdp_t *pdp)
 {
+	struct sgsn_pdp_ctx *pctx = pdp->priv;
+
 	LOGPDPX(DGPRS, LOGL_INFO, pdp, "Context %p was deleted\n", pdp);
 
+	/* unlink the now non-existing library handle from the pdp
+	 * context */
+	pctx->lib = NULL;
+
+	sgsn_ggsn_ctx_drop_pdp(pctx);
 	return 0;
 }