Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.

array_get_element and array_get_slice qualify as leakproof, since
they will silently return NULL for bogus subscripts.  But
array_set_element and array_set_slice throw errors for such cases,
making them clearly not leakproof.  contain_leaked_vars was evidently
written with only the former case in mind, as it gave the wrong answer
for assignment SubscriptingRefs (nee ArrayRefs).

This would be a live security bug, were it not that assignment
SubscriptingRefs can only occur in INSERT and UPDATE target lists,
while we only care about leakproofness for qual expressions; so the
wrong answer can't occur in practice.  Still, that's a rather shaky
answer for a security-related question; and maybe in future somebody
will want to ask about leakproofness of a tlist.  So it seems wise to
fix and even back-patch this correction.

(We would need some change here anyway for the upcoming
generic-subscripting patch, since extensions might make different
tradeoffs about whether to throw errors.  Commit 558d77f20 attempted
to lay groundwork for that by asking check_functions_in_node whether a
SubscriptingRef contains leaky functions; but that idea fails now that
the implementation methods of a SubscriptingRef are not SQL-visible
functions that could be marked leakproof or not.)

Back-patch to 9.6.  While 9.5 has the same issue, the code's a bit
different.  It seems quite unlikely that we'd introduce any actual bug
in the short time 9.5 has left to live, so the work/risk/reward balance
isn't attractive for changing 9.5.

Discussion: https://postgr.es/m/3143742.1607368115@sss.pgh.pa.us

src/backend/optimizer/util/clauses.c

@@ -1121,7 +1121,6 @@ contain_leaked_vars_walker(Node *node, void *context)
 		case T_ScalarArrayOpExpr:
 		case T_CoerceViaIO:
 		case T_ArrayCoerceExpr:
-		case T_SubscriptingRef:
 
 			/*
 			 * If node contains a leaky function call, and there's any Var
@@ -1133,6 +1132,23 @@ contain_leaked_vars_walker(Node *node, void *context)
 				return true;
 			break;
 
+		case T_SubscriptingRef:
+			{
+				SubscriptingRef *sbsref = (SubscriptingRef *) node;
+
+				/*
+				 * subscripting assignment is leaky, but subscripted fetches
+				 * are not
+				 */
+				if (sbsref->refassgnexpr != NULL)
+				{
+					/* Node is leaky, so reject if it contains Vars */
+					if (contain_var_clause(node))
+						return true;
+				}
+			}
+			break;
+
 		case T_RowCompareExpr:
 			{
 				/*