y4m_test: fix segfault if test files are missing

Change-Id: I7a04beb83095e5c0821048909f81f45be8b5eee3

test/y4m_test.cc

diff --git a/test/y4m_test.cc b/test/y4m_test.cc
index a555329..91b30b1 100644
--- a/test/y4m_test.cc
+++ b/test/y4m_test.cc
@@ -140,7 +140,7 @@
 class Y4mVideoWriteTest
     : public Y4mVideoSourceTest {
  protected:
-  Y4mVideoWriteTest() {}
+  Y4mVideoWriteTest() : tmpfile_(NULL) {}
 
   virtual ~Y4mVideoWriteTest() {
     delete tmpfile_;