serde_with: KeyValueMap serialization panics on empty sequence or map entries

Summary

### Summary

The public `KeyValueMap` serializer assumes that each mapped element has at least one field or item to use as the map key, but it subtracts `1` from the caller-visible length before validating that assumption. An application that serializes attacker-controlled data through `#[serde_as(as = "KeyValueMap<_>")]` can be crashed by an empty inner sequence or map entry.

### Details

The affected public surface includes:

- Serialization of `#[serde_as(as = "KeyValueMap<_>")]` values through `serde_json::to_string` or any other Serde serializer` - Public `KeyValueMap` conversions for sequence and map-backed entries`

The root cause is: The `KeyValueMap` serializer preallocating `Vec::with_capacity(len - 1)` or `Vec::with_capacity(len.unwrap_or(17) - 1)` before checking that the element actually contains the required first key field or item.

The vulnerable data/control flow is: attacker-controlled empty entry -> `serde_json::to_string` -> `KeyValueMap<TAs>::serialize_as` -> `SeqAsMapSerializer::{serialize_seq,serialize_map}` -> `Vec::with_capacity(len - 1)` or `Vec::with_capacity(len.unwrap_or(17) - 1)` -> panic

Relevant source locations:

- `serde_with/src/key_value_map.rs:590` - `serde_with/src/key_value_map.rs:599` - `serde_with/src/key_value_map.rs:613` - `serde_with/src/key_value_map.rs:632` - `serde_with/src/key_value_map.rs:648`

### PoC

```rust /* [dependencies] serde = {version = "*", features = ["derive"]} serde_with = "*" serde_json = "*" */

use serde::Serialize; use serde_with::{serde_as, KeyValueMap};

#[derive(Serialize)] #[serde(transparent)] struct Seq(Vec<String>);

#[serde_as] #[derive(Serialize)] #[serde(transparent)] struct KVMap { #[serde_as(as = "KeyValueMap<_>")] foo: Vec<Seq>, }

fn main() { let value = KVMap { foo: vec![Seq(Vec::new())], }; let _ = serde_json::to_string(&value).unwrap(); } ```

### Impact

A local attacker who can trigger serialization of attacker-controlled data through `KeyValueMap` can terminate the process, causing a denial of service.