better-hw-compatibility: fix out of bounds access

better-hw-compatibility: fix out of bounds access

With --enable-better-hw-compatibility an access to array element -1
can be observed for VP9/ActiveMapTest.Test/0
../vp9/encoder/vp9_rdopt.c:3938:53: runtime error:
  index -1 out of bounds for type 'RefBuffer [3]'

There doesn't seem anything that would prevent ref_frame from being 0.
If there is no reference frame it can probably be assumed that it
isn't scaled.

Signed-off-by: Matthias Räncker <theonetruecamper@gmx.de>
Change-Id: I0a29cd0ffc9a19742e5e72203d5ec5d0a16eac7a