tests: only destroy instance if creation succeeded

If we try to destroy a null instance object, the loader segfaults.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
diff --git a/tests/vkrenderframework.cpp b/tests/vkrenderframework.cpp
index 545b904..d3901cf 100644
--- a/tests/vkrenderframework.cpp
+++ b/tests/vkrenderframework.cpp
@@ -196,7 +196,7 @@
 
     // reset the driver
     delete m_device;
-    vkDestroyInstance(this->inst);
+    if (this->inst) vkDestroyInstance(this->inst);
 }
 
 void VkRenderFramework::InitState()