nat: Move the write queue init to the allocation function

This is required for unit tests that want to queue messages
and see if we can provoke a memleak.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 0a0a125..20ddec2 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -676,7 +676,6 @@
 		return -1;
 	}
 
-	write_queue_init(&bsc->write_queue, 100);
 	bsc->write_queue.bfd.data = bsc;
 	bsc->write_queue.bfd.fd = ret;
 	bsc->write_queue.read_cb = ipaccess_bsc_read_cb;
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 28b1bc7..1365427 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -70,6 +70,7 @@
 		return NULL;
 
 	con->nat = nat;
+	write_queue_init(&con->write_queue, 100);
 	return con;
 }