Core dump in range_table_mutator()

Commit 64919aaab made pull_up_simple_subquery set rte->subquery = NULL
after doing the deed, so that we don't waste cycles copying a
now-useless subquery tree around.  I discovered today while
working on another patch that if you invoke query_tree_mutator
or range_table_mutator on the whole Query after that point,
range_table_mutator dumps core, because it's expecting subquery
links to never be NULL.  There's apparently noplace in our core
code that does that today, but I'm a bit surprised we've not heard
complaints from anyone else.  I propose to do this to harden it:

diff --git a/src/backend/nodes/nodeFuncs.c b/src/backend/nodes/nodeFuncs.c
index 876f84dd39..8d58265010 100644
--- a/src/backend/nodes/nodeFuncs.c
+++ b/src/backend/nodes/nodeFuncs.c
@@ -3788,7 +3788,9 @@ range_table_mutator(List *rtable,
                 /* we don't bother to copy eref, aliases, etc; OK? */
                 break;
             case RTE_SUBQUERY:
-                if (!(flags & QTW_IGNORE_RT_SUBQUERIES))
+                /* In the planner, subquery is null if it's been flattened */
+                if (!(flags & QTW_IGNORE_RT_SUBQUERIES) &&
+                    rte->subquery != NULL)
                 {
                     CHECKFLATCOPY(newrte->subquery, rte->subquery, Query);
                     MUTATE(newrte->subquery, newrte->subquery, Query *);


            regards, tom lane