gprs_ns2: const the return value of gprs_ns2_ip_vc_sockaddr / gprs_ns2_ip_bind_sockaddr

The sockaddr should not be changed.
free and create the bind/nsvc if the address should be changed.

Change-Id: I371ac2361b569e36722b02fc9cd82ec8da2fa9e3
diff --git a/src/gb/gprs_ns2_vty.c b/src/gb/gprs_ns2_vty.c
index 55e87ae..92c0095 100644
--- a/src/gb/gprs_ns2_vty.c
+++ b/src/gb/gprs_ns2_vty.c
@@ -247,7 +247,7 @@
 {
 	struct osmo_sockaddr_str remote;
 	struct osmo_sockaddr_str local;
-	struct osmo_sockaddr *sockaddr;
+	const struct osmo_sockaddr *sockaddr;
 
 	switch (nsvc->ll) {
 	case GPRS_NS_LL_UDP: {