aom_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.

cherry-picked from libvpx:
93c823e24 vpx_dsp/get_prob: relocate den == 0 test

Change-Id: Ie0604ad405a97ed754e4b88c6d580eb4894ea0f6
1 file changed