elm_config: save the backend string as stringshare

the string might be on the stack ...

The same as in the previous commit, that path was never really tested,
caused by the fact that the *none* engine was not even running

src/lib/elementary/elm_config.c

@@ -4522,7 +4522,7 @@ elm_config_web_backend_set(const char *backend)
 {
    _elm_config->priv.web_backend = EINA_TRUE;
    if (_elm_web_init(backend))
-     _elm_config->web_backend = backend;
+     eina_stringshare_replace(&_elm_config->web_backend, backend);
 }
 
 EAPI const char *