wwan: don't disconnect if nothing to disconnect

Avoid this error:

NetworkManager[25181]: <warn> (cdc-wdm1): Failed to connect 'T-Mobile Internet': Connection requested IPv4 but IPv4 is unsuported by the modem.
NetworkManager[25181]: <info> (cdc-wdm1): device state change: prepare -> failed (reason 'modem-init-failed') [40 120 28]

** (NetworkManager:25181): CRITICAL **: mm_modem_simple_disconnect: assertion 'MM_IS_MODEM_SIMPLE (self)' failed

self->priv->simple_iface is only valid if stage1/prepare actually
completes, so don't try to access it if stage1/prepare failed.

src/devices/wwan/nm-modem-broadband.c

@@ -866,11 +866,15 @@ simple_disconnect_ready (MMModemSimple *modem_iface,
 }
 
 static void
-disconnect (NMModem *self,
+disconnect (NMModem *modem,
             gboolean warn)
 {
+	NMModemBroadband *self = NM_MODEM_BROADBAND (modem);
 	SimpleDisconnectContext *ctx;
 
+	if (!self->priv->simple_iface)
+		return;
+
 	ctx = g_slice_new (SimpleDisconnectContext);
 	ctx->self = g_object_ref (self);