vp9,read_inter_block_mode_info: quiet msan warning

best_sub8x8[1] won't be used meaningfully when is_compound is false, but
may trigger an msan warning as the value is copied around and later
clamped.

BUG=667044

Change-Id: Icc24c3b72cdb550bebea44d4aaa4ff8bf3fbab56
(cherry picked from commit cb22359d027bc44cf84fa53a3ffd81c098816cc8)

test/invalid_file_test.cc

@@ -144,6 +144,7 @@ const DecodeParam kVP9InvalidFileTests[] = {
   { 1, "invalid-vp90-2-03-size-202x210.webm.ivf.s113306_r01-05_b6-.ivf" },
   { 1,
     "invalid-vp90-2-10-show-existing-frame.webm.ivf.s180315_r01-05_b6-.ivf" },
+  { 1, "invalid-crbug-667044.webm" },
 };
 
 VP9_INSTANTIATE_TEST_CASE(InvalidFileTest,

test/test-data.mk

@@ -777,6 +777,8 @@ LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-vp90-2-07-frame_parallel-2.web
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-vp90-2-07-frame_parallel-3.webm
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-crbug-629481.webm
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-crbug-629481.webm.res
+LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-crbug-667044.webm
+LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += invalid-crbug-667044.webm.res
 
 ifeq ($(CONFIG_DECODE_PERF_TESTS),yes)
 # Encode / Decode test

test/test-data.sha1

@@ -844,3 +844,5 @@ a000d568431d07379dd5a8ec066061c07e560b47 *invalid-vp90-2-00-quantizer-63.ivf.kf_
 5d9474c0309b7ca09a182d888f73b37a8fe1362c *invalid-crbug-629481.webm.res
 7602e00378161ca36ae93cc6ee12dd30b5ba1e1d *vp90-2-22-svc_1280x720_3.ivf
 02e53e3eefbf25ec0929047fe50876acdeb040bd *vp90-2-22-svc_1280x720_3.ivf.md5
+e402cbbf9e550ae017a1e9f1f73931c1d18474e8 *invalid-crbug-667044.webm
+d3964f9dad9f60363c81b688324d95b4ec7c8038 *invalid-crbug-667044.webm.res

vp9/decoder/vp9_decodemv.c

@@ -770,6 +770,10 @@ static void read_inter_block_mode_info(VP9Decoder *const pbi,
     int idx, idy;
     PREDICTION_MODE b_mode;
     int_mv best_sub8x8[2];
+    const uint32_t invalid_mv = 0x80008000;
+    // Initialize the 2nd element as even though it won't be used meaningfully
+    // if is_compound is false, copying/clamping it may trigger a MSan warning.
+    best_sub8x8[1].as_int = invalid_mv;
     for (idy = 0; idy < 2; idy += num_4x4_h) {
       for (idx = 0; idx < 2; idx += num_4x4_w) {
         const int j = idy * 2 + idx;