Fix an encoding failure case when speed features are on

This commit fixes an encoding failure case triggered when early
termination feature is turned on for transform block size search.
It resolves the corresponding enc/dec mismatch issue.

Change-Id: I2c5b7d8b1efe25fe3810e6ed307f4b1865dede49
diff --git a/vp10/encoder/rdopt.c b/vp10/encoder/rdopt.c
index adcd547..dc56563 100644
--- a/vp10/encoder/rdopt.c
+++ b/vp10/encoder/rdopt.c
@@ -912,9 +912,9 @@
       if (cpi->sf.tx_size_search_breakout &&
           (rd == INT64_MAX ||
 #if CONFIG_EXT_TX
-           (s == 1 && tx_type != DCT_DCT) ||
+           (s == 1 && tx_type != DCT_DCT && n < start_tx) ||
 #else
-           (s == 1) ||
+           (s == 1 && n < start_tx) ||
 #endif
            (n < (int) max_tx_size && rd > last_rd)))
         break;