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_fr.c b/src/gb/gprs_ns2_fr.c
index 7b9450c..a7455d7 100644
--- a/src/gb/gprs_ns2_fr.c
+++ b/src/gb/gprs_ns2_fr.c
@@ -101,7 +101,8 @@
 
 static void free_vc(struct gprs_ns2_vc *nsvc)
 {
-	OSMO_ASSERT(nsvc);
+	if (!nsvc)
+		return;
 
 	if (!nsvc->priv)
 		return;