Quickfix for a segfault with file input via -i.

Stuff like

  ./bin/sigrok-cli -i rnd.dd -a transitioncounter

would segfault (rnd.dd consists of random bytes) because device->plugin
was NULL and was being dereferenced.

Thanks Olivier Fauchon <olivier@aixmarseille.com> for reporting.

device.c

@@ -235,6 +235,9 @@ gboolean sr_device_has_hwcap(struct sr_device *device, int hwcap)
 {
 	int *capabilities, i;
 
+	if (!device || !device->plugin)
+		return;
+
 	if ((capabilities = device->plugin->get_capabilities()))
 		for (i = 0; capabilities[i]; i++)
 			if (capabilities[i] == hwcap)