Fix COPY FROM ON_ERROR SET_NULL with selective column list

Fix COPY FROM ON_ERROR SET_NULL with selective column list

When using COPY FROM ... ON_ERROR SET_NULL with a selective column list, the
domain_with_constraint array was incorrectly allocated based on the length of
the target column list. While the array was populated sequentially,
CopyFromTextLikeOneRow attempted to access it using the physical attribute
index (attnum - 1). This mismatch caused out-of-bounds reads when targeting
high-numbered columns, allowing NULL values to bypass NOT NULL domain checks
and be silently inserted.

Fix by allocating the array to match the total number of physical attributes
(num_phys_attrs) and indexing via attnum - 1, bringing it into alignment with
other per-column arrays in BeginCopyFrom.

Author: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDdej0c0gWJi2FnbirzhgzyZNPiTwC1P5B_-dSNCzq-91A@mail.gmail.com