Fix variable lifespan in ExecInitCoerceToDomain().

This undoes a mistake in 1ec7679f1: domainval and domainnull were
meant to live across loop iterations, but they were incorrectly
moved inside the loop.  The effect was only to emit useless extra
EEOP_MAKE_READONLY steps, so it's not a big deal; nonetheless,
back-patch to v13 where the mistake was introduced.

Ranier Vilela

Discussion: https://postgr.es/m/CAEudQAqXuhbkaAp-sGH6dR6Nsq7v28_0TPexHOm6FiDYqwQD-w@mail.gmail.com

src/backend/executor/execExpr.c

@@ -3137,6 +3137,8 @@ ExecInitCoerceToDomain(ExprEvalStep *scratch, CoerceToDomain *ctest,
 					   ExprState *state, Datum *resv, bool *resnull)
 {
 	DomainConstraintRef *constraint_ref;
+	Datum	   *domainval = NULL;
+	bool	   *domainnull = NULL;
 	ListCell   *l;
 
 	scratch->d.domaincheck.resulttype = ctest->resulttype;
@@ -3183,8 +3185,6 @@ ExecInitCoerceToDomain(ExprEvalStep *scratch, CoerceToDomain *ctest,
 	foreach(l, constraint_ref->constraints)
 	{
 		DomainConstraintState *con = (DomainConstraintState *) lfirst(l);
-		Datum	   *domainval = NULL;
-		bool	   *domainnull = NULL;
 		Datum	   *save_innermost_domainval;
 		bool	   *save_innermost_domainnull;