Use palloc_array() in a few more places to avoid overflow

These could overflow on 32-bit systems.

Backpatch-through: 14
Security: CVE-2026-6473

contrib/hstore_plperl/hstore_plperl.c

@@ -121,7 +121,7 @@ plperl_to_hstore(PG_FUNCTION_ARGS)
 
 	pcount = hv_iterinit(hv);
 
-	pairs = palloc(pcount * sizeof(Pairs));
+	pairs = palloc_array(Pairs, pcount);
 
 	i = 0;
 	while ((he = hv_iternext(hv)))

contrib/hstore_plpython/hstore_plpython.c

@@ -149,7 +149,7 @@ plpython_to_hstore(PG_FUNCTION_ARGS)
 		Py_ssize_t	i;
 		Pairs	   *pairs;
 
-		pairs = palloc(pcount * sizeof(*pairs));
+		pairs = palloc_array(Pairs, pcount);
 
 		for (i = 0; i < pcount; i++)
 		{