In postgres_fdw, don't try to ship MULTIEXPR updates to remote server.

In a statement like "UPDATE remote_tab SET (x,y) = (SELECT ...)",
we'd conclude that the statement could be directly executed remotely,
because the sub-SELECT is in a resjunk tlist item that's not examined
for shippability.  Currently that ends up crashing if the sub-SELECT
contains any remote Vars.  Prevent the crash by deeming MULTIEXEC
Params to be unshippable.

This is a bit of a brute-force solution, since if the sub-SELECT
*doesn't* contain any remote Vars, the current execution technology
would work; but that's not a terribly common use-case for this syntax,
I think.  In any case, we generally don't try to ship sub-SELECTs, so
it won't surprise anybody that this doesn't end up as a remote direct
update.  I'd be inclined to see if that general limitation can be fixed
before worrying about this case further.

Per report from Lukáš Sobotka.

Back-patch to 9.6.  9.5 had MULTIEXPR, but we didn't try to perform
remote direct updates then, so the case didn't arise anyway.

Discussion: https://postgr.es/m/CAJif3k+iA_ekBB5Zw2hDBaE1wtiQa4LH4_JUXrrMGwTrH0J01Q@mail.gmail.com

contrib/postgres_fdw/deparse.c

@@ -390,6 +390,22 @@ foreign_expr_walker(Node *node,
 			{
 				Param	   *p = (Param *) node;
 
+				/*
+				 * If it's a MULTIEXPR Param, punt.  We can't tell from here
+				 * whether the referenced sublink/subplan contains any remote
+				 * Vars; if it does, handling that is too complicated to
+				 * consider supporting at present.  Fortunately, MULTIEXPR
+				 * Params are not reduced to plain PARAM_EXEC until the end of
+				 * planning, so we can easily detect this case.  (Normal
+				 * PARAM_EXEC Params are safe to ship because their values
+				 * come from somewhere else in the plan tree; but a MULTIEXPR
+				 * references a sub-select elsewhere in the same targetlist,
+				 * so we'd be on the hook to evaluate it somehow if we wanted
+				 * to handle such cases as direct foreign updates.)
+				 */
+				if (p->paramkind == PARAM_MULTIEXPR)
+					return false;
+
 				/*
 				 * Collation rule is same as for Consts and non-foreign Vars.
 				 */