qtdemux: turn impossible condition into an assert

qtdemux: turn impossible condition into an assert

qtdemux_update_streams() is only ever called after checking
`qtdemux->streams_aware` is TRUE. There is no need to check for that
condition again.

`qtdemux->streams_aware` is only modified when the demuxer is
hard-resetted, which is mutually exclusive with demuxing, so it cannot
be modified during the call.

https://bugzilla.gnome.org/show_bug.cgi?id=797191