vp9_scale_references: fail if no free buffer is available

If `get_free_fb()` fails to return a buffer for use with scaling, fail
immediately. This avoids heap overflows (read) in code that assumes the
frame has been scaled to the current resolution
(`vp9_int_pro_motion_estimation()` and `combined_motion_search()`).

A test case will be added in a follow up.

This matches the behavior in libaom since:
  4a8c004b80 Move the assertion back to get_free_fb().
  ...
  Also change the scale_references() function to not ignore
  get_free_fb() failure.

Bug: 487259772
Change-Id: If8028e40173b06b67cc161735af730eeb99ac57e