Fix crash in short link processor (#13839)

Fixes #13819

modules/markup/html.go

@@ -651,6 +651,7 @@ func shortLinkProcessorFull(ctx *postProcessCtx, node *html.Node, noLink bool) {
 			// When parsing HTML, x/net/html will change all quotes which are
 			// not used for syntax into UTF-8 quotes. So checking val[0] won't
 			// be enough, since that only checks a single byte.
+			if len(val) > 1 {
 				if (strings.HasPrefix(val, "“") && strings.HasSuffix(val, "”")) ||
 					(strings.HasPrefix(val, "‘") && strings.HasSuffix(val, "’")) {
 					const lenQuote = len("‘")
@@ -662,6 +663,7 @@ func shortLinkProcessorFull(ctx *postProcessCtx, node *html.Node, noLink bool) {
 					const lenQuote = len("‘")
 					val = val[1 : len(val)-lenQuote]
 				}
+			}
 			props[key] = val
 		}
 	}