make some compilers happy

Commit f10e4f8 causes some old compilers to complain about implicit
return from non-void function. A false positive error but make the
compiler happy so it stops complaining.

src/env_universal_common.cpp

@@ -1385,6 +1385,8 @@ std::unique_ptr<universal_notifier_t> universal_notifier_t::new_notifier_for_str
             return make_unique<universal_notifier_named_pipe_t>(test_path);
         }
     }
+    DIE("should never reach this statement");
+    return NULL;
 }
 
 // Default implementations.