[PATCH] bugfix for int2vectorin

I believe the int2vectorin function should handle invalid input more cleanly.

Current behavior:

-- Correct input format:
SELECT '1 2 3'::int2vector;
int2vector
------------
1 2 3
(1 row)

-- Three variations on incorrect input format
SELECT '1not 2really_an 3int2vector'::int2vector;
int2vector
------------
1 2 3
(1 row)

SELECT '1,2,3'::int2vector;
int2vector
------------
1
(1 row)

SELECT '1, 2,3'::int2vector;
int2vector
------------
1 2
(1 row)

-- What other types do:
SELECT '1,2'::oid;
ERROR: invalid input syntax for type oid: "1,2"

Behavior after attached patch:

-- Correct input format:
SELECT '1 2 3'::int2vector;
int2vector
------------
1 2 3
(1 row)

-- Three variations on incorrect input format
SELECT '1not 2really_an 3int2vector'::int2vector;
ERROR: invalid input for int2vector: "1not 2really_an 3int2vector"

SELECT '1,2,3'::int2vector;
ERROR: invalid input for int2vector: "1,2,3"

SELECT '1, 2,3'::int2vector;
ERROR: invalid input for int2vector: "1, 2,3"

-- What other types do:
SELECT '1,2'::oid;
ERROR: invalid input syntax for type oid: "1,2"

Regards,
Caleb