vpx_dsp/get_prob: relocate den == 0 test

to get_binary_prob(). the only other caller mode_mv_merge_probs() does
its own test on 0.

BUG=chromium:639712

Change-Id: I1178688706baeca2883f7aadbc254abb219a44ce
(cherry picked from commit 93c823e24b7b5b91de729217075c08e9082b80bd)
1 file changed