- ebml-read: add some sanity checks when going from 64-bit
  to 32-bit length
- matroska-ids: codec_data_size has been checked via
  gst_ebml_read_binary(), is existing allocation.
- matroska-demux: alloc size is from existing allocations

g_memdup() is deprecated since GLib 2.68 and we want to avoid
deprecation warnings with recent versions of GLib.

Also use gst_buffer_new_memdup() instead of _wrapped(g_memdup(),..).

Part-of: <gstreamer/gst-plugins-good!993>