Fix: check sum header fields for validity

The rsync vulnerability CVE-2024-12084 also affected gokrazy/rsync
(before this commit): An attacker could send an invalid sum header
and thereby trigger a panic (but no heap buffer overflow!) like:

    panic: runtime error: slice bounds out of range [:512] with length 16
    goroutine 277 [running]:
    github.com/gokrazy/rsync/rsyncd.(*sendTransfer).receiveSums(0xc0000d7b68)
        /home/michael/go/src/github.com/gokrazy/rsync/rsyncd/sender.go:136 +0x339

After this commit, the code no longer panics but sends an error:

    gokr-rsync [sender]: invalid checksum length 512

types.go

@@ -1,6 +1,10 @@
 package rsync
 
-import "github.com/gokrazy/rsync/internal/rsyncwire"
+import (
+	"fmt"
+
+	"github.com/gokrazy/rsync/internal/rsyncwire"
+)
 
 // rsync/rsync.h:struct sum_buf
 type SumBuf struct {
@@ -32,26 +36,43 @@ type SumHead struct {
 }
 
 func (sh *SumHead) ReadFrom(c *rsyncwire.Conn) error {
+	// TODO(protocol>=30): update maxBlockLen
+	const maxBlockLen = 1 << 29 // see rsync.h:OLD_MAX_BLOCK_SIZE
+
 	var err error
 	sh.ChecksumCount, err = c.ReadInt32()
 	if err != nil {
 		return err
 	}
+	if sh.ChecksumCount < 0 {
+		return fmt.Errorf("invalid checksum count %d", sh.ChecksumCount)
+	}
 
 	sh.BlockLength, err = c.ReadInt32()
 	if err != nil {
 		return err
 	}
+	if sh.BlockLength < 0 || sh.BlockLength > maxBlockLen {
+		return fmt.Errorf("invalid block length %d", sh.BlockLength)
+	}
 
 	sh.ChecksumLength, err = c.ReadInt32()
 	if err != nil {
 		return err
 	}
+	// TODO(protocol>=27): update max sh.ChecksumLength check
+	if sh.ChecksumLength < 0 || sh.ChecksumLength > 16 {
+		return fmt.Errorf("invalid checksum length %d", sh.ChecksumLength)
+	}
 
 	sh.RemainderLength, err = c.ReadInt32()
 	if err != nil {
 		return err
 	}
+	if sh.RemainderLength < 0 || sh.RemainderLength > sh.BlockLength {
+		return fmt.Errorf("invalid remainder length %d", sh.RemainderLength)
+	}
+
 	return nil
 }