Fix uninitialized context use case in supertx and var-tx

This commit fixes the use of uninitialized context values in the
combination of supertx and var-tx.

Change-Id: I2d36badf5c9806ea402ce3e19515cc299e6b79e8

vp10/encoder/encodemb.c

@@ -1277,7 +1277,11 @@ void vp10_encode_sb_supertx(MACROBLOCK *x, BLOCK_SIZE bsize) {
 
   for (plane = 0; plane < MAX_MB_PLANE; ++plane) {
     const struct macroblockd_plane* const pd = &xd->plane[plane];
+#if CONFIG_VAR_TX
+    const TX_SIZE tx_size = TX_4X4;
+#else
     const TX_SIZE tx_size = plane ? get_uv_tx_size(mbmi, pd) : mbmi->tx_size;
+#endif
     vp10_subtract_plane(x, bsize, plane);
     vp10_get_entropy_contexts(bsize, tx_size, pd,
                               ctx.ta[plane], ctx.tl[plane]);