dashdemux: Fix leak in gst_dash_demux_stream_update_fragment_info()

dashdemux: Fix leak in gst_dash_demux_stream_update_fragment_info()

The gst_dash_demux_stream_update_fragment_info function could call
gst_dash_demux_stream_update_headers_info function twice. The
gst_dash_demux_stream_update_headers_info function will set header_uri and
index_uri to some newly allocated strings. The values set by the first call of
gst_dash_demux_stream_update_headers_info will leak when the function is
called for a second time.

The solution is to call gst_adaptive_demux_stream_fragment_clear before the
second call of gst_dash_demux_stream_update_headers_info

https://bugzilla.gnome.org/show_bug.cgi?id=753188