mssdemux: only add live fragments after manifest fragments

mssdemux: only add live fragments after manifest fragments

A live manifest may have a set (> LookAheadFragmentCount) of fragments
that have already been served and are stored on the server, maybe
indefinitely.  Adding the parsed live fragments after the manifest
fragments breaks duration reporting and the seekable range.

Fix by only adding parsed fragments outside the list of fragments which
assumes that the fragment list in the manifest is accurate enough to not
stray too far off what's in the retrieved data.

https://bugzilla.gnome.org/show_bug.cgi?id=779447