tests: Initialize fields so test does not crash.

I had forgotten to define the LD_LIBRARY_PATH and VK_LAYER_PATH
environment variables in VS2015 before running the tests. The
test crashed when it tried to delete the un-initialized pointers.
I have added a default initialization to VK_NULL_HANDLE so you
get an error message instead of a crash.

Change-Id: Ia8e2770914d10dc67be744e418dafe82cc164d67
diff --git a/tests/vkrenderframework.cpp b/tests/vkrenderframework.cpp
index d933a0b..545625a 100644
--- a/tests/vkrenderframework.cpp
+++ b/tests/vkrenderframework.cpp
@@ -38,7 +38,8 @@
     }
 
 VkRenderFramework::VkRenderFramework()
-    : m_commandPool(), m_commandBuffer(), m_renderPass(VK_NULL_HANDLE),
+    : inst(VK_NULL_HANDLE), m_device(NULL), m_commandPool(VK_NULL_HANDLE),
+      m_commandBuffer(NULL), m_renderPass(VK_NULL_HANDLE),
       m_framebuffer(VK_NULL_HANDLE), m_width(256.0), // default window width
       m_height(256.0),                               // default window height
       m_render_target_fmt(VK_FORMAT_R8G8B8A8_UNORM),