cryptobyte: reject negative Unwrite argument

Fixes golang/go#57112

Change-Id: I7a533046a6451d7ae3704eb81e6ddeec8442cf06
GitHub-Last-Rev: 3b088d9
GitHub-Pull-Request: golang#249
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/464338
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

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")
 	}