gtphub: fix numerous segfaults, and other stupidities.

Initialize llist_heads to empty (2 were missing). Move those for struct gtphub
instances to gtphub_zero() (one moved, one added).

In from_[gs]gsns_read_cb(), use a return type that can actually reflect
negative return values.

resolved_addr.buf: no need to take the address of a byte array var
(cosmetic).

Pass the proper user data address to sgsn_ares_query(), not the address of
the pointer holding the user data address.

Initialize ggsn_lookup->expiry_entry (was missing). Publish the function for that
in gtphub.h so gtphub_ext.c can use it.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/src/gprs/gtphub.c b/openbsc/src/gprs/gtphub.c
index b3638fe..f0c96bc 100644
--- a/openbsc/src/gprs/gtphub.c
+++ b/openbsc/src/gprs/gtphub.c
@@ -718,6 +718,8 @@
 static void gtphub_zero(struct gtphub *hub)
 {
 	ZERO_STRUCT(hub);
+	INIT_LLIST_HEAD(&hub->ggsn_lookups);
+	INIT_LLIST_HEAD(&hub->resolved_ggsns);
 }
 
 static int gtphub_sock_init(struct osmo_fd *ofd,
@@ -1243,7 +1245,7 @@
 	struct osmo_sockaddr from_addr;
 	struct osmo_sockaddr to_addr;
 	struct osmo_fd *to_ofd;
-	size_t len;
+	int len;
 	uint8_t *reply_buf;
 
 	len = gtphub_read(from_ggsns_ofd, &from_addr, buf, sizeof(buf));
@@ -1497,7 +1499,7 @@
 	struct osmo_sockaddr from_addr;
 	struct osmo_sockaddr to_addr;
 	struct osmo_fd *to_ofd;
-	size_t len;
+	int len;
 	uint8_t *reply_buf;
 
 	len = gtphub_read(from_sgsns_ofd, &from_addr, buf, sizeof(buf));
@@ -1796,8 +1798,6 @@
 {
 	gtphub_zero(hub);
 
-	INIT_LLIST_HEAD(&hub->resolved_ggsns);
-
 	expiry_init(&hub->expire_seq_maps, GTPH_SEQ_MAPPING_EXPIRY_SECS);
 	expiry_init(&hub->expire_tei_maps, GTPH_TEI_MAPPING_EXPIRY_MINUTES * 60);