Validate client input to prevent panics from malformed transactions

A malicious or buggy client could send transaction fields with the wrong
length and trigger runtime panics (slice/index out of range, slice-to-array
conversion, nil deref) in the parsing and handler code. These were caught by
the connection-level recover, so they dropped the client connection and dumped
a stack trace to stdout rather than crashing the process, but they are still
incorrect behavior, a log-flood vector, and a latent crash if the recover
scope ever changes.

Fix at the source and harden the safety net:

- Add ClientIDFromBytes / ChatIDFromBytes helpers that return ok=false on a
  length mismatch, and use them in the transaction handlers instead of direct
  [2]byte(...) / [4]byte(...) conversions on field data. Nil-check ClientMgr.Get
  results, and length-guard FieldOptions and the HandleUpdateUser sub-field
  header. Malformed input now yields an error reply (or a clean no-op for
  reply-less handlers) instead of panicking.
- Bounds-check FileResumeData.UnmarshalBinary (header length and fork count)
  and guard the ForkInfoList[0] accesses against an empty list.
- Bounds-check FlatFileInformationFork parsing (reachable on upload): validate
  the fixed header, name, and comment lengths, and fix a latent 72+nameSize
  uint16 overflow. Route Write through UnmarshalBinary.
- panic.go: stop printing stack traces to stdout (keep structured logging) so a
  client cannot flood stdout by repeatedly triggering a panic.
- handleTransaction: recover per-transaction so one malformed request no longer
  tears down the whole client connection.

Adds tests for the new helpers, the hardened resume-data and flat-file-object
decoders, and handler-level malformed-ID handling.

1 files changed, 4 insertions, 2 deletions

diff --git a/hotline/file_transfer.go b/hotline/file_transfer.go
index e5898db..98999f1 100644
--- a/hotline/file_transfer.go
+++ b/hotline/file_transfer.go
@@ -246,7 +246,7 @@ func (fh *FileHeader) Read(p []byte) (int, error) {
 
 func DownloadHandler(w io.Writer, fullPath string, fileTransfer *FileTransfer, fs FileStore, rLogger *slog.Logger, preserveForks bool) error {
 	var dataOffset int64
-	if fileTransfer.FileResumeData != nil {
+	if fileTransfer.FileResumeData != nil && len(fileTransfer.FileResumeData.ForkInfoList) > 0 {
 		dataOffset = int64(binary.BigEndian.Uint32(fileTransfer.FileResumeData.ForkInfoList[0].DataSize[:]))
 	}
 
@@ -458,7 +458,9 @@ func DownloadFolderHandler(rwc io.ReadWriter, fullPath string, fileTransfer *Fil
 			if err := frd.UnmarshalBinary(resumeDataBytes); err != nil {
 				return err
 			}
-			dataOffset = int64(binary.BigEndian.Uint32(frd.ForkInfoList[0].DataSize[:]))
+			if len(frd.ForkInfoList) > 0 {
+				dataOffset = int64(binary.BigEndian.Uint32(frd.ForkInfoList[0].DataSize[:]))
+			}
 		case DlFldrActionNextFile:
 			// client asked to skip this file
 			return nil