postgres_fdw: Avoid sharing list substructure.

list_concat(list_concat(a, b), c) destructively changes both a and b;
to avoid such perils, copy lists of remote_conds before incorporating
them into larger lists via list_concat().

Ashutosh Bapat, per a report from Etsuro Fujita

contrib/postgres_fdw/postgres_fdw.c

@@ -3488,30 +3488,30 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 	{
 		case JOIN_INNER:
 			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
-											   fpinfo_i->remote_conds);
+										  list_copy(fpinfo_i->remote_conds));
 			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
-											   fpinfo_o->remote_conds);
+										  list_copy(fpinfo_o->remote_conds));
 			break;
 
 		case JOIN_LEFT:
 			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
-											  fpinfo_i->remote_conds);
+										  list_copy(fpinfo_i->remote_conds));
 			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
-											   fpinfo_o->remote_conds);
+										  list_copy(fpinfo_o->remote_conds));
 			break;
 
 		case JOIN_RIGHT:
 			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
-											  fpinfo_o->remote_conds);
+										  list_copy(fpinfo_o->remote_conds));
 			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
-											   fpinfo_i->remote_conds);
+										  list_copy(fpinfo_i->remote_conds));
 			break;
 
 		case JOIN_FULL:
 			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
-											  fpinfo_i->remote_conds);
+										  list_copy(fpinfo_i->remote_conds));
 			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
-											  fpinfo_o->remote_conds);
+										  list_copy(fpinfo_o->remote_conds));
 			break;
 
 		default: