fix AS OF clause panic for certain expressions

enginetest/queries/queries.go

@@ -10751,6 +10751,10 @@ var ErrorQueries = []QueryErrorTest{
 		Query:       "SELECT i FROM myhistorytable AS OF MAX(i)",
 		ExpectedErr: sql.ErrInvalidAsOfExpression,
 	},
+	{
+		Query:          "SELECT i FROM myhistorytable AS OF (SELECT 1)",
+		ExpectedErrStr: "invalid AS OF expression type",
+	},
 	{
 		Query:       "SELECT pk FROM one_pk WHERE pk > ?",
 		ExpectedErr: sql.ErrUnboundPreparedStatementVariable,

sql/planbuilder/show.go

@@ -615,6 +615,8 @@ func (b *Builder) buildAsOfExpr(inScope *scope, time ast.Expr) sql.Expression {
 			err := sql.ErrInvalidAsOfExpression.New(v)
 			b.handleErr(err)
 		}
+	case *ast.Subquery:
+		b.handleErr(fmt.Errorf("invalid AS OF expression type"))
 	default:
 	}
 	return b.buildScalar(b.newScope(), time)