Fix build with GCC 6 due to lifetime issues.

--- a/googletest/src/gtest.cc
+++ b/googletest/src/gtest.cc
@@ -2654,10 +2654,12 @@
     test->Run();
   }
 
-  // Deletes the test object.
-  impl->os_stack_trace_getter()->UponLeavingGTest();
-  internal::HandleExceptionsInMethodIfSupported(
-      test, &Test::DeleteSelf_, "the test fixture's destructor");
+  if (test != NULL) {
+    // Deletes the test object.
+    impl->os_stack_trace_getter()->UponLeavingGTest();
+    internal::HandleExceptionsInMethodIfSupported(
+        test, &Test::DeleteSelf_, "the test fixture's destructor");
+  }
 
   result_.set_elapsed_time(internal::GetTimeInMillis() - start);