Properly initialize variable.

Commit 3bc7dafa9b forgot to do this.

Noted while experimenting with valgrind.

src/backend/optimizer/path/joinpath.c

@@ -756,7 +756,7 @@ sort_inner_and_outer(PlannerInfo *root,
 	JoinType	save_jointype = jointype;
 	Path	   *outer_path;
 	Path	   *inner_path;
-	Path	   *cheapest_partial_outer;
+	Path	   *cheapest_partial_outer = NULL;
 	Path	   *cheapest_safe_inner = NULL;
 	List	   *all_pathkeys;
 	ListCell   *l;