gprs_ns_sns: Properly initialize sockaddr_in in gprs_nsvc_create_ip4()

When putting together a sockaddr_in, we must not only set the IP
address and port, but also set the address family to AF_INET.  And
while at it, let's zero-initialize the entire 'struct sockdadr_in'.

Change-Id: I1c8d8fe7f79a2ec737baa7800247269c3271983e
diff --git a/src/gb/gprs_ns_sns.c b/src/gb/gprs_ns_sns.c
index cbaa468..dd8d3a2 100644
--- a/src/gb/gprs_ns_sns.c
+++ b/src/gb/gprs_ns_sns.c
@@ -88,6 +88,8 @@
 	struct gprs_nsvc *nsvc;
 	struct sockaddr_in sin;
 	/* copy over. Both data structures use network byte order */
+	memset(&sin, 0, sizeof(sin));
+	sin.sin_family = AF_INET;
 	sin.sin_addr.s_addr = ip4->ip_addr;
 	sin.sin_port = ip4->udp_port;