x/crypto/cryptobyte: reject negative Unwrite argument

Fixes golang/go#57112

cryptobyte/builder.go

@@ -303,9 +303,9 @@ func (b *Builder) add(bytes ...byte) {
 	b.result = append(b.result, bytes...)
 }
 
-// Unwrite rolls back n bytes written directly to the Builder. An attempt by a
-// child builder passed to a continuation to unwrite bytes from its parent will
-// panic.
+// Unwrite rolls back non-negative n bytes written directly to the Builder.
+// An attempt by a child builder passed to a continuation to unwrite bytes
+// from its parent will panic.
 func (b *Builder) Unwrite(n int) {
 	if b.err != nil {
 		return
@@ -317,6 +317,9 @@ func (b *Builder) Unwrite(n int) {
 	if length < 0 {
 		panic("cryptobyte: internal error")
 	}
+	if n < 0 {
+		panic("cryptobyte: attempted to unwrite negative number of bytes")
+	}
 	if n > length {
 		panic("cryptobyte: attempted to unwrite more than was written")
 	}