baylibre-acme: Properly free GErrors returned by g_file_get_contents()
When g_file_get_contents() encounters an error a new GError will be
allocated and passed back to the application. The application is
responsible for freeing this GError.
The baylibre-acme driver currently does not do this and as a result leaks
memory during the scan process when no device is found.
Add the missing g_error_free() invocations to fix the issue.