If fdopen(3) fails then ctx->output_file is NULL and should not be used
in cleanup_test_output() called next.
Problem detected by the Coverity tool
CID 1042543 (#1 of 1): Dereference after null check (FORWARD_NULL)9.
var_deref_model: Passing "ctx" to function
"cleanup_test_output(libusbx_testlib_ctx *)", which dereferences null
"ctx->output_file".
* cleanup_test_output() produced an assertion error when compiled with MSVC
due to closing ctx->output_file
* set ctx->output_file to stdout by default
