test/vp[89]_boolcoder_test: quiet msan warnings

the bitreaders may fill beyond what was written to the buffer as an
optimization. the data isn't used meaningfully, but it may trigger a
msan warning.

BUG=b/140939146

Change-Id: Id03cd203b8ee7ecaf6fdfe3f3c9f2ccfec527129

test/vp9_boolcoder_test.cc

@@ -66,6 +66,9 @@ TEST(VP9, TestBitIO) {
         }
 
         vpx_stop_encode(&bw);
+        // vpx_reader_fill() may read into uninitialized data that
+        // isn't used meaningfully, but may trigger an MSan warning.
+        memset(bw_buffer + bw.pos, 0, sizeof(BD_VALUE) - 1);
 
         // First bit should be zero
         GTEST_ASSERT_EQ(bw_buffer[0] & 0x80, 0);