Initialize reject_cause to a valid number

Currently starting with the opencfg.cfg.nanobts and writing it out
and then trying to start again will not work. The network reject_cause
is initialized to 0 which is not a valid reject reason and when writing
this to the config file and trying to parse it will fail.

Pick roaming not allowed as a harmless option to those phones
accidently trying to connect to the BTS.
diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index 291d407..446a820 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -216,6 +216,7 @@
 	net->country_code = country_code;
 	net->network_code = network_code;
 	net->num_bts = 0;
+	net->reject_cause = GSM48_REJECT_ROAMING_NOT_ALLOWED;
 	net->T3101 = GSM_T3101_DEFAULT;
 	net->T3113 = GSM_T3113_DEFAULT;
 	/* FIXME: initialize all other timers! */