Use g_memdup2() where available and add fallback for older GLib versions

Use g_memdup2() where available and add fallback for older GLib versions

- x264 encoder bitrate profile manager: alloc size is based
  on existing allocation
- asfdemux: change length var to 64-bit and check for G_MAXUINT
- realmedia: opaque_data_len is read from 32 bits and then
  only subtracted upon.

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

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-ugly/-/merge_requests/83>