Fix NPE in fuzzer (#16680) (#16682)

The fuzzer found an issue with the issue pattern processor where there is a spurious
path.Clean which does not need to be there. This PR also sets the default AppURL for
the fuzzer too.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>

modules/markup/html_test.go

@@ -5,6 +5,7 @@
 package markup_test
 
 import (
+	"io"
 	"strings"
 	"testing"
 
@@ -526,3 +527,18 @@ func BenchmarkEmojiPostprocess(b *testing.B) {
 		assert.NoError(b, err)
 	}
 }
+
+func TestFuzz(t *testing.T) {
+	s := "t/l/issues/8#/../../a"
+	renderContext := RenderContext{
+		URLPrefix: "https://example.com/go-gitea/gitea",
+		Metas: map[string]string{
+			"user": "go-gitea",
+			"repo": "gitea",
+		},
+	}
+
+	err := PostProcess(&renderContext, strings.NewReader(s), io.Discard)
+
+	assert.NoError(t, err)
+}