Fix exception in av1_tpl_get_frame_importance()

BUG=aomedia:3420

Change-Id: I2f68fa96cc9e0c51ef76bf6b70f4a21927f3149c

av1/encoder/tpl_model.c

diff --git a/av1/encoder/tpl_model.c b/av1/encoder/tpl_model.c
index c09f0d7..55a2b27 100644
--- a/av1/encoder/tpl_model.c
+++ b/av1/encoder/tpl_model.c
@@ -2063,6 +2063,7 @@
           RDCOST(tpl_frame->base_rdmult, this_stats->mc_dep_rate,
                  this_stats->mc_dep_dist);
       double dist_scaled = (double)(this_stats->recrf_dist << RDDIV_BITS);
+      dist_scaled = AOMMAX(dist_scaled, 1);
       intra_cost_base += log(dist_scaled) * cbcmp;
       mc_dep_cost_base += log(dist_scaled + mc_dep_delta) * cbcmp;
       cbcmp_base += cbcmp;