Fix seeking HLS on-demand streaming not working

This issue occurs when playing HLS streaming delivered
from a server that does not support partial requests.
The issue is reproduced as follows(using Ubuntu 20.04 PC):

1. Prepare HLS example content.

$ mkdir test
$ ffmpeg -i example.flac -vn -c:a aac -b:a 128000 -f hls -hls_list_size 0 test/output.m3u8
(ffmpeg 4.2.4 is used)

2. Prepare web server without partial requests support.
(Docker version 20.10.12 and NGINX official Docker image is used)

$ docker run --name tmp-nginx-container -d nginx
$ docker cp tmp-nginx-container:/etc/nginx/conf.d/default.conf .
$ docker rm -f tmp-nginx-container

Edit default.conf and add "max_ranges 0;" to "location / {...}".
This disables partial requests support,
removes 'Accept-Ranges: bytes' header from the server response.
Then, run the server:

$ docker run --name test-nginx -v $PWD/test:/usr/share/nginx/html:ro -v $PWD/default.conf:/etc/nginx/conf.d/default.conf -d -p 8080:80 nginx

3. Setup MPD to Play the next URL.

http://address-of-the-server:8080/output.m3u8

Seeking this stream results in "exception: Not seekable".

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -533,9 +533,8 @@ FfmpegDecode(DecoderClient &client, InputStream *input,
 		: FromFfmpegTimeChecked(format_context.duration, AV_TIME_BASE_Q);
 
 	client.Ready(audio_format,
-		     input
-		     ? input->IsSeekable()
-		     : IsSeekable(format_context),
+		     (input ? input->IsSeekable() : false)
+		     || IsSeekable(format_context),
 		     total_time);
 
 	FfmpegParseMetaData(client, format_context, audio_stream);