go/doc/comment: don't crash if there are no words to wrap

Fixes #52783

Change-Id: I12e3da40d49cd92776052bf19cb07ee8a07f3ee0
Reviewed-on: https://go-review.googlesource.com/c/go/+/405215
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/go/doc/comment/testdata/crash1.txt

@@ -0,0 +1,16 @@
+-- input --
+[]
+
+[]: http://
+-- gofmt --
+[]
+
+[]: http://
+-- html --
+<p><a href="http://"></a>
+-- markdown --
+[](http://)
+-- text --
+
+
+[]: http://

src/go/doc/comment/text.go

@@ -141,7 +141,7 @@ func (p *textPrinter) text(out *bytes.Buffer, indent string, x []Text) {
 	p.long.Reset()
 
 	var seq []int
-	if p.width < 0 {
+	if p.width < 0 || len(words) == 0 {
 		seq = []int{0, len(words)} // one long line
 	} else {
 		seq = wrap(words, p.width-utf8.RuneCountInString(indent))