Handle the case where a single-line comment is less than 3 characters long.

mojom/mojom_parser/formatter/printer.go

Index: mojom/mojom_parser/formatter/printer.go
diff --git a/mojom/mojom_parser/formatter/printer.go b/mojom/mojom_parser/formatter/printer.go
index 821defa5602180b0a02ea896ba1b065f949929b4..6de06c3c2e80b2fc60eee83fb434b0f5f2de99f5 100644
--- a/mojom/mojom_parser/formatter/printer.go
+++ b/mojom/mojom_parser/formatter/printer.go
@@ -664,8 +664,7 @@ func (p *printer) writeSingleLineComment(comment lexer.Token) {
 
 	// We expect that the first 2 characters are // followed by a space or tab.
 	// If the third character is not a space or tab, we insert a space.
-	space := commentText[2]
-	if space != ' ' && space != '\t' {
+	if len(commentText) > 2 && commentText[2] != ' ' && commentText[2] != '\t' {
 		commentText = "// " + commentText[2:]
 	}
 	p.write(commentText)