ns2: Use NULL and not '0' when returning a NULL-Pointer

Change-Id: I9458e6dcf8133fbcfbc89437e58aad049a6d432b
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index 21a45a1..57a08d1 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -216,7 +216,7 @@
 	struct osmo_sockaddr_str remote_str;
 
 	if (!buf_len)
-		return 0;
+		return NULL;
 
 	switch (nsvc->ll) {
 	case GPRS_NS_LL_UDP: