Fix federated.federatedx_create_handler with --ps-protocol, for CLX-77

CLX-77 code used "thd->lex->result!=NULL" to check if the SELECT has
an INTO part. This is not correct, as the condition also holds when
we're using the PS protocol.
Use a more generic check: check whether the SELECT has any side effect.

storage/federatedx/federatedx_pushdown.cc

@@ -185,10 +185,11 @@ create_federatedx_select_handler(THD* thd, SELECT_LEX *sel)
   /*
     Currently, ha_federatedx_select_handler::init_scan just takes the 
     thd->query and sends it to the backend.
-    This obviously won't work if the SELECT has an INTO part.
-    Refuse to work in this case.
+    This obviously won't work if the SELECT uses an "INTO @var" or
+    "INTO OUTFILE". It is also unlikely to work if the select has some
+    other kind of side effect.
   */
-  if (thd->lex->result)
+  if (sel->uncacheable & UNCACHEABLE_SIDEEFFECT)
     return NULL;
 
   /*