Prevent division-by-zero

Change-Id: Id606669875ef1dcb163ff7dec619dae6110d9047

av1/encoder/firstpass.c

@@ -2794,6 +2794,7 @@ static void find_next_key_frame(AV1_COMP *cpi, FIRSTPASS_STATS *this_frame) {
       boost_score += (decay_accumulator * frame_boost);
     }
   }
+  if (loop_decay_counter > 0)
     av_decay_accumulator /= (double)loop_decay_counter;
 
   reset_fpf_position(twopass, start_position);