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 <[e-mail]>
Reviewed-by: Rae Moar <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/kunit/string-stream.c

···
 {
 	KUNIT_STATIC_STUB_REDIRECT(string_stream_destroy, stream);
 
-	if (!stream)
+	if (IS_ERR_OR_NULL(stream))
 		return;
 
 	string_stream_clear(stream);