httplive: have some sane max_duration_length
httplive: have some sane max_duration_length

Regression from ac3f61953b25a61cf46c16badcfd2a24f890b83e as without this
there could be negative/zero max_duration_length if playlist didn't
change and it would cause instantenious refetch of that playlist.

krichter722/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc