avformat/hls: properly take stream_index into account when seeking

Properly take stream_index into account so that a keyframe will be
looked for in the specified stream_index only.

Similarly, only check timestamp validity against the specified
stream_index.

Also remove code for stream_index == -1 case which does not actually
happen as it is handled by generic code.

This is based on an initial patch by James Deng.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
