cdba-server: Don't close what wasn't opened

In the case that we quit without actually having opened a device this
would dereference a NULL pointer, so check the device before doing so.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

cdba-server.c

@@ -413,7 +413,8 @@ int main(int argc, char **argv)
 
 done:
 
-	device_close(selected_device);
+	if (selected_device)
+		device_close(selected_device);
 
 	return 0;
 }