qtdemux: fix parsing in push mode when moov atom is at the end

qtdemux: fix parsing in push mode when moov atom is at the end

When playing an mp4 file with the MOOV atom at the end of the file, playback
fails with the error message "no 'moov' atom within the first 10 MB". This is
due to a mistake in the upstream_size typing, making the seek to the end of
file never happening.

https://bugzilla.gnome.org/show_bug.cgi?id=684972