Fix compile warning in rdopt.c

Silence a spurious warning that uv_tx "may be used uninitialized"

Change-Id: Ia7b930dca6ce16701d14302bfafa9fcf50b44ed8

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 130da4c..60b453b 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -9591,7 +9591,7 @@
             best_model_rd_palette = INT64_MAX;
     int skippable = 0, rate_overhead_palette = 0;
     RD_STATS rd_stats_y;
-    TX_SIZE uv_tx;
+    TX_SIZE uv_tx = TX_4X4;
     uint8_t *const best_palette_color_map =
         x->palette_buffer->best_palette_color_map;
     uint8_t *const color_map = xd->plane[0].color_index_map;