Modify boxnum to fix degenerate size videos

Fixes a memory issue and computes values correctly for 1x1 and
1xn or nx1 videos.
This also fixes a test failure in AV1FrameSizeTestsLarge.OneByOneVideo.

Change-Id: Ib5a4efd094763fb6ceb119383c11f776784f58a4
1 file changed