Parse panics with "index out of range [0]" when an integer element such as (0002,0000) or Rows has length 0 #392

Summary

dicom.Parse panics on a 140-byte file whose File Meta Information Group Length (0002,0000) is UL with a value length of 0. The empty UL decodes to an empty []int, and readHeader takes [0] from it without checking the length (read.go:230):

metaLen := maybeMetaLen.Value.GetValue().([]int)[0]

Nothing recovers the panic, so any program that parses files it did not create with dicom.Parse crashes instead of getting an error.

Reproduction (main at 8ae5137)

package main

import (
	"bytes"
	"fmt"

	"github.com/suyashkumar/dicom"
)

func main() {
	var b bytes.Buffer
	b.Write(make([]byte, 128)) // preamble
	b.WriteString("DICM")
	// (0002,0000) VR=UL, length 0
	b.Write([]byte{0x02, 0x00, 0x00, 0x00, 'U', 'L', 0x00, 0x00})
	data := b.Bytes()
	_, err := dicom.Parse(bytes.NewReader(data), int64(len(data)), nil)
	fmt.Println("err =", err)
}

panic: runtime error: index out of range [0] with length 0

github.com/suyashkumar/dicom.(*reader).readHeader(...)
github.com/suyashkumar/dicom.NewParser(...)
github.com/suyashkumar/dicom.parseInternal(...)
github.com/suyashkumar/dicom.Parse(...)

Second site

readNativeFrames does the same with MustGetInts(...)[0] for Rows, Columns, BitsAllocated and SamplesPerPixel (read.go:446-454). A file with a valid header and an empty US Rows and Columns before PixelData panics there the same way. read.go:433 (NumberOfFrames) and write.go:322 index the same way, but I haven't produced a panic from either.

Expected

An error, the same as for other malformed headers. ErrorMetaElementGroupLength looks like the right one for read.go:230.

Fix

I have a fix ready. An empty (0002,0000) is treated like a missing one, so it returns ErrorMetaElementGroupLength unless AllowMissingMetaElementGroupLength is set. readNativeFrames reads Rows, Columns, BitsAllocated and SamplesPerPixel through a small helper that returns an error for an empty value. A new TestParse_EmptyIntElements covers both sites and panics without the fix, and make test passes with it. I'll open a PR if that sounds right to you.

(Found by fuzzing and reproduced with the help of an AI coding assistant.)