Fix another palloc in critical section.

Also add a regression test for a GIN index with enough items with the same
key, so that a GIN posting tree gets created. Apparently none of the
existing GIN tests were large enough for that.

This code is new, no backpatching required.

src/test/regress/sql/create_index.sql

@@ -636,6 +636,20 @@ RESET enable_seqscan;
 RESET enable_indexscan;
 RESET enable_bitmapscan;
 
+--
+-- Try a GIN index with a lot of items with same key. (GIN creates a posting
+-- tree when there are enough duplicates)
+--
+CREATE TABLE array_gin_test (a int[]);
+
+INSERT INTO array_gin_test SELECT ARRAY[1, g%5, g] FROM generate_series(1, 10000) g;
+
+CREATE INDEX array_gin_test_idx ON array_gin_test USING gin (a);
+
+SELECT COUNT(*) FROM array_gin_test WHERE a @> '{2}';
+
+DROP TABLE array_gin_test;
+
 --
 -- HASH
 --