Turbopack: fix source map panic (#81212)
Fix a panic, related to scope hoisting.
```
An error occurred while generating the chunk item X
Caused by:
- Cannot decode dummy BytePos
```
```
thread 'tokio-runtime-worker' panicked at turbopack/crates/turbopack-ecmascript/src/lib.rs:2431:13:
Cannot decode dummy BytePos
stack backtrace:
0: __rustc::rust_begin_unwind
at /rustc/59aa1e873028948faaf8b97e5e02d4db340ad7b1/library/std/src/panicking.rs:697:5
1: core::panicking::panic_fmt
at /rustc/59aa1e873028948faaf8b97e5e02d4db340ad7b1/library/core/src/panicking.rs:75:14
2: <turbopack_ecmascript::CodeGenResultComments>::decode_bytepos
at next.js/turbopack/crates/turbopack-ecmascript/src/lib.rs:2431:13
3: <turbopack_ecmascript::CodeGenResultCommentsConsumable as swc_common::comments::Comments>::has_leading
at next.js/turbopack/crates/turbopack-ecmascript/src/lib.rs:2499:21
4: <swc_ecma_codegen::Emitter<swc_ecma_codegen::text_writer::basic_impl::JsWriter<&mut alloc::vec::Vec<u8>>, turbopack_ecmascript::CodeGenResultSourceMap>>::has_leading_comment
at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/swc_ecma_codegen-15.0.1/src/lib.rs:839:24
5: <swc_ecma_codegen::Emitter<swc_ecma_codegen::text_writer::basic_impl::JsWriter<&mut alloc::vec::Vec<u8>>, turbopack_ecmascript::CodeGenResultSourceMap>>::has_leading_comment
at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/swc_ecma_codegen-15.0.1/src/lib.rs:830:29
```
