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_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;
 }