Zero segment counter before accumulating.

The segment counts are computed as part of packing the bitstream,
so they might have been computed already in the recode loop. Zero
the accumulator to avoid double counting.

This fixes some encoder/decoder mismatches.

Change-Id: Ib7816034cbbb1db41101116b706302b02fad3a2c

vp10/encoder/segmentation.c

@@ -317,6 +317,9 @@ void vp10_choose_segmap_coding_method(VP10_COMMON *cm, MACROBLOCKD *xd) {
 
   (void) xd;
 
+  // We are about to recompute all the segment counts, so zero the accumulators.
+  vp10_zero(cm->counts.seg);
+
   // First of all generate stats regarding how well the last segment map
   // predicts this one
   for (tile_row = 0; tile_row < cm->tile_rows; tile_row++) {