relcache: Avoid memory leak on tables with no CHECK constraints

relcache: Avoid memory leak on tables with no CHECK constraints

As complained about by Valgrind, in commit a379061a22a8 I failed to
realize that I was causing rd_att->constr->check to become allocated
when no CHECK constraints exist; previously it'd remain NULL.  (This was
my bug, not the mentioned commit author's).  Fix by making the
allocation conditional, and set ->check to NULL if unallocated.

Reported-by: Yasir <yasir.hussain.shah@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202505082025.57ijx3qrbx7u@alvherre.pgsql