Fix incorrect column name in error message for range partition bound check

Hi all,

Recently, while working with partitioned tables, I identified an issue where the error message references an incorrect column name when a range-partitioned table’s boundary values include MINVALUE or MAXVALUE. 

For example:

postgres=# CREATE TABLE pt_test_colname(a int, b int, c int) PARTITION BY RANGE(a, b);

-- right case

postgres=# CREATE TABLE pt_test_colname_p1 PARTITION OF pt_test_colname FOR VALUES FROM (10, now()) TO (100, 100);

ERROR: specified value cannot be cast to type integer for column "b"

LINE 1: ...PARTITION OF pt_test_colname FOR VALUES FROM (10, now()) TO ...

                                                             ^

-- wrong case

postgres=# CREATE TABLE pt_test_colname_p1 PARTITION OF pt_test_colname FOR VALUES FROM (minvalue, now()) TO (100, 100);

ERROR: specified value cannot be cast to type integer for column "a"

LINE 1: ...ION OF pt_test_colname FOR VALUES FROM (minvalue, now()) TO ...

                                                             ^

The datetime value returned by now() cannot be cast to the partition key b (integer type), yet the error message incorrectly attributes this casting failure to column a when the preceding boundary value is the special MINVALUE or MAXVALUE.