sqlsmith bug hunt: panic on btrim

[neverchanje]

Describe the bug
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2022-06-20T16:34:34.476353Z ERROR risingwave_batch::rpc::service::task_service: failed to fire task internal error: [PlanNode: Ok(Project(ProjectNode { select_list: [ExprNode { expr_type: Trim, return_type: Some(DataType { type_name: Varchar, precision: 0, scale: 0, is_nullable: true, interval_type: Invalid, field_type: [] }), rex_node: Some(FuncCall(FunctionCall { children: [ExprNode { expr_type: ConstantValue, return_type: Some(DataType { type_name: Varchar, precision: 0, scale: 0, is_nullable: true, interval_type: Invalid, field_type: [] }), rex_node: Some(Constant(ConstantValue { body: [54, 70, 89, 78, 87, 104, 78, 117, 118, 48] })) }, ExprNode { expr_type: ConstantValue, return_type: Some(DataType { type_name: Varchar, precision: 0, scale: 0, is_nullable: true, interval_type: Invalid, field_type: [] }), rex_node: Some(Constant(ConstantValue { body: [97, 98, 99] })) }] })) }] }))] Failed to build executor: Expr error: Internal(children.len() == 1

Stack backtrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/bb8c2f41174caceec00c28bc6c5c20ae9f9a175c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/bb8c2f41174caceec00c28bc6c5c20ae9f9a175c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/bb8c2f41174caceec00c28bc6c5c20ae9f9a175c/library/std/src/backtrace.rs:328:13
   3: anyhow::kind::Adhoc::new
             at /Users/wutao/.cargo/registry/src/github.com-1ecc6299db9ec823/anyhow-1.0.57/src/kind.rs:70:36
   4: risingwave_expr::expr::build_expr_from_prost::build_trim_expr
             at ./src/expr/src/expr/build_expr_from_prost.rs:103:5

To Reproduce
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select trim(Both 'abc' from '6FYNWhNuv0');

Expected behavior
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Additional context
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