Fix #4081 Check for leading / in base before removing it (#4083)

Author: sapk

modules/markup/html.go

@@ -129,8 +129,8 @@ func URLJoin(base string, elems ...string) string {
 		return ""
 	}
 	joinedURL := baseURL.ResolveReference(argURL).String()
-	if !baseURL.IsAbs() {
-		return joinedURL[1:] // Removing leading '/'
+	if !baseURL.IsAbs() && !strings.HasPrefix(base, "/") {
+		return joinedURL[1:] // Removing leading '/' if needed
 	}
 	return joinedURL
 }

modules/markup/html_test.go

@@ -85,6 +85,12 @@ func TestURLJoin(t *testing.T) {
 			"a/", "b/c/", "/../d/"),
 		newTest("https://try.gitea.io/a/b/c#d",
 			"https://try.gitea.io", "a/b", "c#d"),
+		newTest("/a/b/d",
+			"/a/", "b/c/", "/../d/"),
+		newTest("/a/b/c",
+			"/a", "b/c/"),
+		newTest("/a/b/c#hash",
+			"/a", "b/c#hash"),
 	} {
 		assert.Equal(t, test.Expected, URLJoin(test.Base, test.Elements...))
 	}