Initialize both luma and chroma planes in palette encoding

This avoids the use of uninitialized value in palette encoding.

BUG=aomedia:568

Change-Id: I1000b28d9acf25a70ef641d4494f0b116a4ca071

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 9452a7b..ccf70a2 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -3676,6 +3676,7 @@
 #endif  // CONFIG_FILTER_INTRA
 #if CONFIG_PALETTE
   pmi->palette_size[0] = 0;
+  pmi->palette_size[1] = 0;
   if (above_mi)
     palette_y_mode_ctx +=
         (above_mi->mbmi.palette_mode_info.palette_size[0] > 0);