Set unused reference frames to first ref

Set unused reference frames to first ref

If a reference frame is not referenced, then set the index for that
reference to the first one used/referenced instead of unused slot.
Unused slot means key frame, as key frame resets all slots with itself.

This CL extracts `get_first_ref_frame()` from `reset_fb_idx_unused()`
with a typo fixing, and sets all unused reference frames to first ref in
vp9 uncompressed header.

Bug: webrtc:13442
Change-Id: I99523bc2ceedf27efe376d1113851ff342982181