to_char() on integer with V format silently wraps on overflow

Hi, All!

to_char() with a V format multiplies the value by 10^n. For integer
input that multiply is a bare int32 expression. On overflow the
product wraps and to_char() prints the wrapped digits. There is no
error. The same call on bigint is correct. Plain integer multiply
already raises "integer out of range".

SELECT to_char(3, '9V999999999');
-- actual: -1294967296
-- expected: ERROR: integer out of range

SELECT to_char(2147483647, '9V9');
-- actual: -10
-- expected: ERROR: integer out of range

SELECT to_char(3::bigint, '9V999999999');
-- 3000000000

SELECT 3 * 1000000000;
-- ERROR: integer out of range

Cause: int4_to_char() does value * (int32) pow(10, multi) with no
overflow check. int8_to_char() already uses int8mul and dtoi8.

The attached patch uses int4mul and dtoi4 for the same step with regress
tests.

Thoughts?

--
Regards,
Rachitskiy Andrey