relcache: Avoid memory leak on tables with no CHECK constraints

As complained about by Valgrind, in commit a379061a22 I failed to
realize that I was causing rd_att->constr->check to become allocated
when no CHECK constraints exist; previously it'd remain NULL.  (This was
my bug, not the mentioned commit author's).  Fix by making the
allocation conditional, and set ->check to NULL if unallocated.

Reported-by: Yasir <yasir.hussain.shah@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202505082025.57ijx3qrbx7u@alvherre.pgsql

src/backend/utils/cache/relcache.c

@@ -4598,10 +4598,13 @@ CheckNNConstraintFetch(Relation relation)
 	HeapTuple	htup;
 	int			found = 0;
 
-	/* Allocate array with room for as many entries as expected */
-	check = (ConstrCheck *)
-		MemoryContextAllocZero(CacheMemoryContext,
-							   ncheck * sizeof(ConstrCheck));
+	/* Allocate array with room for as many entries as expected, if needed */
+	if (ncheck > 0)
+		check = (ConstrCheck *)
+			MemoryContextAllocZero(CacheMemoryContext,
+								   ncheck * sizeof(ConstrCheck));
+	else
+		check = NULL;
 
 	/* Search pg_constraint for relevant entries */
 	ScanKeyInit(&skey[0],