abis_nm: Consider a STATE CHG without avail_status as an empty avail_status

At least some nanoBTS 139 send the last even (going online) as
'enabled' 'unlocked' but no avail status IE (which I guess mean an empty
set, the doc 12.21 isn't that clear about that).

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index 7a67fdf..8789674 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -813,7 +813,8 @@
 			new_state.availability = *TLVP_VAL(&tp, NM_ATT_AVAIL_STATUS);
 		DEBUGPC(DNM, "AVAIL=%s(%02x) ", nm_avail_name(new_state.availability),
 			new_state.availability);
-	}
+	} else
+		new_state.availability = 0xff;
 	if (TLVP_PRESENT(&tp, NM_ATT_ADM_STATE)) {
 		new_state.administrative = *TLVP_VAL(&tp, NM_ATT_ADM_STATE);
 		DEBUGPC(DNM, "ADM=%2s ", nm_adm_name(new_state.administrative));