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/plan/subquery.go

@@ -349,6 +349,10 @@ func (s *Subquery) evalMultiple(ctx *sql.Context, row sql.Row) ([]interface{}, e
 		return nil, err
 	}
 
+	if s.b == nil {
+		return nil, fmt.Errorf("attempted to evaluate uninitialized subquery")
+	}
+
 	iter, err := s.b.Build(ctx, q, row)
 	if err != nil {
 		return nil, err
@@ -434,6 +438,10 @@ func (s *Subquery) HasResultRow(ctx *sql.Context, row sql.Row) (bool, error) {
 		return false, err
 	}
 
+	if s.b == nil {
+		return false, fmt.Errorf("attempted to evaluate uninitialized subquery")
+	}
+
 	iter, err := s.b.Build(ctx, q, row)
 	if err != nil {
 		return false, err

sql/planbuilder/show.go

@@ -615,7 +615,10 @@ func (b *Builder) buildAsOfExpr(inScope *scope, time ast.Expr) sql.Expression {
 			err := sql.ErrInvalidAsOfExpression.New(v)
 			b.handleErr(err)
 		}
+	case *ast.ConvertExpr:
+	case ast.InjectedExpr:
 	default:
+		b.handleErr(fmt.Errorf("invalid AS OF expression type"))
 	}
 	return b.buildScalar(b.newScope(), time)
 }