tests: fix a few msvc warnings

Change-Id: I7ee89d6fc2afc20563d1784598ce83e29c0e6651
diff --git a/test/test_vector_test.cc b/test/test_vector_test.cc
index 4adf9af..53b7636 100644
--- a/test/test_vector_test.cc
+++ b/test/test_vector_test.cc
@@ -35,7 +35,7 @@
 
   void OpenMD5File(const std::string& md5_file_name_) {
     md5_file_ = libvpx_test::OpenTestDataFile(md5_file_name_);
-    ASSERT_TRUE(md5_file_) << "Md5 file open failed. Filename: "
+    ASSERT_TRUE(md5_file_ != NULL) << "Md5 file open failed. Filename: "
         << md5_file_name_;
   }