dashdemux: correctly handle an HTTP-XSDATE that is exactly the size of the date string

dashdemux: correctly handle an HTTP-XSDATE that is exactly the size of the date string

The code in the gst_dash_demux_parse_http_xsdate() was trying to
handle the case where the string is not null terminated by resizing
the buffer and appending a zero byte. This does not work if the buffer
is exactly the length of the string because the gst_buffer_resize()
function does not re-allocate the buffer, it just changes its size.

If a buffer is passed to gst_dash_demux_parse_http_xsdate() that is
exactly the length of the string, the function fails with an assert
failure in gst_buffer_resize().

https://bugzilla.gnome.org/show_bug.cgi?id=762148