kunit: string-stream: Allow ERR_PTR to be passed to string_stream_destroy()

Check the stream pointer passed to string_stream_destroy() for
IS_ERR_OR_NULL() instead of only NULL.

Whatever alloc_string_stream() returns should be safe to pass
to string_stream_destroy(), and that will be an ERR_PTR.

It's obviously good practise and generally helpful to also check
for NULL pointers so that client cleanup code can call
string_stream_destroy() unconditionally - which could include
pointers that have never been set to anything and so are NULL.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>