gtp: cb_delete_context(): Clarify why pdp->lib is set to NULL

Change-Id: I02c46062d6dc9802d9cae057587e66a68a65436c
diff --git a/src/gprs/sgsn_libgtp.c b/src/gprs/sgsn_libgtp.c
index 88b8d04..79d5543 100644
--- a/src/gprs/sgsn_libgtp.c
+++ b/src/gprs/sgsn_libgtp.c
@@ -649,8 +649,9 @@
 
 	LOGPDPX(DGPRS, LOGL_INFO, pdp, "Context %p was deleted\n", pdp);
 
-	/* unlink the now non-existing library handle from the pdp
-	 * context */
+	/* unlink the now non-existing library handle from the pdp context.
+	   This way we avoid calling pdp_freepdp() on it, since after returning
+	   from cb_delete_context callback, libgtp is already doing so. */
 	pctx->lib = NULL;
 
 	sgsn_ggsn_ctx_drop_pdp(pctx);