Tighten an overbroad and misleading assertion.
(Reported by Jim Jewett.)
diff --git a/Objects/setobject.c b/Objects/setobject.c
index ed3d190..3541ff1 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -445,7 +445,7 @@
 		}
 #ifdef Py_DEBUG
 		else
-			assert(entry->key == NULL || entry->key == dummy);
+			assert(entry->key == NULL);
 #endif
 	}