opt/execbuilder: fix buggy allocation of result columns

This commit fixes a subtle bug where a slice was always allocated
with zero capacity, rather than the known required capacity. This bug
caused no user facing issues, but did cause extra allocations when
appending to the slice.

Release note: None

Author: mgartner

pkg/sql/opt/exec/execbuilder/relational.go

@@ -1051,8 +1051,9 @@ func (b *Builder) buildProject(prj *memo.ProjectExpr) (execPlan, error) {
 	}
 
 	var res execPlan
-	exprs := make(tree.TypedExprs, 0, len(projections)+prj.Passthrough.Len())
-	cols := make(colinfo.ResultColumns, 0, len(exprs))
+	numExprs := len(projections) + prj.Passthrough.Len()
+	exprs := make(tree.TypedExprs, 0, numExprs)
+	cols := make(colinfo.ResultColumns, 0, numExprs)
 	ctx := input.makeBuildScalarCtx()
 	for i := range projections {
 		item := &projections[i]