Fix coding performance drop in var-tx, supertx, and ext-partition

This commit fixes the use of uninitialized value in the RD search
loop. It brings back the coding performance loss when the above
three experiments are all turned on.

BUG=aomedia:92

Change-Id: I19d7904c354e7a94e5f672bf416267971791c371

av1/encoder/rdopt.c

@@ -3515,7 +3515,13 @@ static void select_tx_type_yrd(const AV1_COMP *cpi, MACROBLOCK *x,
   const int n4 = 1 << (num_pels_log2_lookup[bsize] - 4);
   int idx, idy;
   int prune = 0;
+  const int count32 = 1 << (2 * (cpi->common.mib_size_log2 -
+                                 mi_width_log2_lookup[BLOCK_32X32]));
+#if CONFIG_EXT_PARTITION
+  RD_STATS rd_stats_stack[16];
+#else
   RD_STATS rd_stats_stack[4];
+#endif
 #if CONFIG_EXT_TX
   int ext_tx_set = get_ext_tx_set(max_tx_size, bsize, is_inter);
 #endif  // CONFIG_EXT_TX
@@ -3529,7 +3535,8 @@ static void select_tx_type_yrd(const AV1_COMP *cpi, MACROBLOCK *x,
 
   av1_invalid_rd_stats(rd_stats);
 
-  for (idx = 0; idx < 4; ++idx) av1_invalid_rd_stats(&rd_stats_stack[idx]);
+  for (idx = 0; idx < count32; ++idx)
+    av1_invalid_rd_stats(&rd_stats_stack[idx]);
 
   for (tx_type = DCT_DCT; tx_type < TX_TYPES; ++tx_type) {
     RD_STATS this_rd_stats;