Initialize unused ExprEvalStep fields.

ExecPushExprSlots didn't initialize ExprEvalStep's resvalue/resnull
steps as it didn't use them. That caused wrong valgrind warnings for
an upcoming patch, so zero-intialize.

Also zero-initialize all scratch ExprEvalStep's allocated on the
stack, to avoid issues with similar future omissions of non-critial
data.

src/backend/executor/execExpr.c

@@ -118,7 +118,7 @@ ExprState *
 ExecInitExpr(Expr *node, PlanState *parent)
 {
 	ExprState  *state;
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 
 	/* Special case: NULL expression produces a NULL ExprState pointer */
 	if (node == NULL)
@@ -155,7 +155,7 @@ ExprState *
 ExecInitExprWithParams(Expr *node, ParamListInfo ext_params)
 {
 	ExprState  *state;
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 
 	/* Special case: NULL expression produces a NULL ExprState pointer */
 	if (node == NULL)
@@ -204,7 +204,7 @@ ExprState *
 ExecInitQual(List *qual, PlanState *parent)
 {
 	ExprState  *state;
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 	List	   *adjust_jumps = NIL;
 	ListCell   *lc;
 
@@ -353,7 +353,7 @@ ExecBuildProjectionInfo(List *targetList,
 {
 	ProjectionInfo *projInfo = makeNode(ProjectionInfo);
 	ExprState  *state;
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 	ListCell   *lc;
 
 	projInfo->pi_exprContext = econtext;
@@ -638,7 +638,7 @@ static void
 ExecInitExprRec(Expr *node, ExprState *state,
 				Datum *resv, bool *resnull)
 {
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 
 	/* Guard against stack overflow due to overly complex expressions */
 	check_stack_depth();
@@ -2273,7 +2273,10 @@ ExecInitExprSlots(ExprState *state, Node *node)
 static void
 ExecPushExprSlots(ExprState *state, LastAttnumInfo *info)
 {
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
+
+	scratch.resvalue = NULL;
+	scratch.resnull = NULL;
 
 	/* Emit steps as needed */
 	if (info->last_inner > 0)
@@ -2659,7 +2662,7 @@ static void
 ExecInitCoerceToDomain(ExprEvalStep *scratch, CoerceToDomain *ctest,
 					   ExprState *state, Datum *resv, bool *resnull)
 {
-	ExprEvalStep scratch2;
+	ExprEvalStep scratch2 = {0};
 	DomainConstraintRef *constraint_ref;
 	Datum	   *domainval = NULL;
 	bool	   *domainnull = NULL;
@@ -2811,7 +2814,7 @@ ExecBuildAggTrans(AggState *aggstate, AggStatePerPhase phase,
 {
 	ExprState  *state = makeNode(ExprState);
 	PlanState  *parent = &aggstate->ss.ps;
-	ExprEvalStep scratch;
+	ExprEvalStep scratch = {0};
 	int			transno = 0;
 	int			setoff = 0;
 	bool		isCombine = DO_AGGSPLIT_COMBINE(aggstate->aggsplit);