Initialize all the refs in MaskedSADx4Test

This will silent valgrind warning.

BUG=aomedia:2603

Change-Id: Icb017fc31d45977917f2c7be7ad7733e8572ad46

test/masked_sad_test.cc

diff --git a/test/masked_sad_test.cc b/test/masked_sad_test.cc
index a281739..aa4dd83 100644
--- a/test/masked_sad_test.cc
+++ b/test/masked_sad_test.cc
@@ -189,6 +189,9 @@
     for (int j = 0; j < MAX_SB_SIZE * MAX_SB_SIZE; j++) {
       src_ptr[j] = rnd.Rand8();
       ref_ptr[j] = rnd.Rand8();
+      (ref_ptr + kBlockSize)[j] = rnd.Rand8();
+      (ref_ptr + 2 * kBlockSize)[j] = rnd.Rand8();
+      (ref_ptr + 3 * kBlockSize)[j] = rnd.Rand8();
       second_pred_ptr[j] = rnd.Rand8();
       msk_ptr[j] = ((rnd.Rand8() & 0x7f) > 64) ? rnd.Rand8() & 0x3f : 64;
       assert(msk_ptr[j] <= 64);