gprs_ns2: allow to use free_vc() with NULL

Usually talloc_free() and other free functions in osmocom allows
to be called with NULL which is then ignored.

Change-Id: If7b0c6916a29d4611d0a40c388414076eb83e6b5
diff --git a/src/gb/gprs_ns2_udp.c b/src/gb/gprs_ns2_udp.c
index 1037b19..2a335c1 100644
--- a/src/gb/gprs_ns2_udp.c
+++ b/src/gb/gprs_ns2_udp.c
@@ -73,6 +73,9 @@
 
 static void free_vc(struct gprs_ns2_vc *nsvc)
 {
+	if (!nsvc)
+		return;
+
 	if (!nsvc->priv)
 		return;