Fix bug in clearing of virtual tuple slot.

I broke/typoed this in 4da597edf1. Astonishingly this mostly
doesn't cause breakage, except when trying to change the tuple
descriptor of a slot (because TTS_FLAG_FIXED is assumed to be set).

Author: Andres Freund

src/backend/executor/execTuples.c

@@ -114,7 +114,7 @@ tts_virtual_clear(TupleTableSlot *slot)
 		pfree(vslot->data);
 		vslot->data = NULL;
 
-		slot->tts_flags = ~TTS_FLAG_SHOULDFREE;
+		slot->tts_flags &= ~TTS_FLAG_SHOULDFREE;
 	}
 
 	slot->tts_nvalid = 0;