hls: only seek if there is an offset

If there is no #EXT-X-BYTERANGE specified, there is no need to seek.
Seeking fails anyway for rtmp, because this protocol does not support
url_seek.

This fixes CNN.m3u from trac ticket 4797 (i.e. Debian bug #798189).

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>