Also fix a mismatch for resizing with ext-refs enabled.

There are various preconditions which need to be true for it to
be valid to set cm->use_prev_frame_mvs = 1, including that the
sizes of this frame and cm->prev_frame must be equal.

With ext-refs enabled, we would sometimes decide to change
cm->prev_frame to point to the LAST_FRAME reference, without
re-checking the preconditions. If the LAST_FRAME was smaller
than the current frame, this could lead to reading garbage off the
end of its mv array, and eventually to an encode/decode mismatch.

We fix this by rewriting the preconditions as checks on
cm->prev_frame directly (rather than using cm->last_width and
cm->last_height), and by testing the preconditions after any
possible adjustment.

This should not affect the bitstream unless ext-refs is enabled,
but may affect the bitstream with ext-refs even if resizing is
not used.

BUG=aomedia:521

Change-Id: I7dfd9ba82cdf77acc2e27e0c9f3aee21d6afeb54