libvpx

get_ref_frame: fix test for valid buffer.

In the current implementation of the encoder,
frame buffers may come from the wider set of
12 such buffers, and is not restricted to the
8 allowed as reference frames. This is only
an implementation detail and does not affect
the constraint of having a total of 8 reference
buffers overall.

Change-Id: I075f777146c2df49c275d89232933f8127235175