COPY FROM ON_ERROR SET_NULL bypasses domain NOT NULL with partial column list

HI hackers,

domain_with_constraint[] was allocated with list_length(attnumlist)
elements and indexed sequentially via foreach_current_index(), but
copyfromparse.c accesses it via attnum - 1 (physical attribute index).
With a partial column list targeting high-numbered columns, this caused
an out-of-bounds read that bypassed domain NOT NULL checks, silently
inserting NULL into NOT NULL domain columns.

Fix by allocating with num_phys_attrs and indexing by attnum - 1,
consistent with all other per-column arrays in BeginCopyFrom().

Patch is attached, and added a new test case to cover this scenario.

Repro:

CREATE DOMAIN d_notnull_int AS int NOT NULL;
CREATE TABLE t (
c1 text, c2 text, c3 text, c4 text, c5 text,
c6 text, c7 text, c8 text, c9 text,
c10 d_notnull_int
);

COPY t(c1, c10) FROM stdin WITH (on_error set_null);
hello bad
\.

SELECT c10 IS NULL FROM t;

Thanks,
Satya