Unreferences the bytes, and returns a pointer the same byte data contents.
As an optimization, the byte data is returned without copying if this was
the last reference to bytes and bytes was created with g_bytes_new(),
g_bytes_new_take() or g_byte_array_free_to_bytes(). In all other cases the
data is copied.
Available since: 2.32
Location to place the length of the returned data.
The argument will be set by the function.