Palette Throughput - don't copy garbage

BUG=aomedia:697

Change-Id: Id7e971e515c9faa3df32e162befd57907278549d

av1/decoder/detokenize.c

diff --git a/av1/decoder/detokenize.c b/av1/decoder/detokenize.c
index 0e95341..f7cab90 100644
--- a/av1/decoder/detokenize.c
+++ b/av1/decoder/detokenize.c
@@ -254,7 +254,7 @@
   }
   // Copy last column to extra columns.
   if (cols < plane_block_width) {
-    for (int i = 0; i < plane_block_height; ++i) {
+    for (int i = 0; i < rows; ++i) {
       memset(color_map + i * plane_block_width + cols,
              color_map[i * plane_block_width + cols - 1],
              (plane_block_width - cols));