The probability training tool generates all CDF_PROB_TOP table if
the counts are all zero. Although it means such cases are never
used in actual encoding, in rd cost pre-calculation, we still
sometimes run into such cases and the token cost calcualtion
function cannot handle all-CDF_PROB_TOP cdfs (Will trigger assertion
when debug mode is on).

Change-Id: I259620e210fea0f6e1f7848724fa006fa06da226