tests: use vpx_codec_dec_cfg_t() to initialize vars

0-initializes [1] and removes a warning for missing initializer fields

[1] http://en.cppreference.com/w/cpp/language/value_initialization

Change-Id: I364248010c8fa663c71d8f06a9999c730e92db4c

test/external_frame_buffer_test.cc

@@ -285,7 +285,7 @@ class ExternalFrameBufferTest : public ::testing::Test {
     video_->Init();
     video_->Begin();
 
-    vpx_codec_dec_cfg_t cfg = {0};
+    vpx_codec_dec_cfg_t cfg = vpx_codec_dec_cfg_t();
     decoder_ = new libvpx_test::VP9Decoder(cfg, 0);
     ASSERT_TRUE(decoder_ != NULL);
   }