riff: create palette_data buffer correctly

riff: create palette_data buffer correctly

gst_buffer_copy_into() will append to any existing
memory region, so don't create a buffer and alloc
some memory, but just create an empty buffer and
let _copy_into() append the memory we want. Fixes
the palette being 2048 bytes with the first half
being filled with garbage.

https://bugzilla.gnome.org/show_bug.cgi?id=686046