Fix build with GCC 6 due to lifetime issues.

--- a/googletest/src/gtest.cc
+++ b/googletest/src/gtest.cc
@@ -2693,10 +2693,12 @@
     test->Run();
   }
 
+  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);