Prevent NPE on invalid diff (#17197)

* Prevent NPE on invalid diff

If ParseCompareInfo returns a nil compare info the defer function needs to ensure
that it does not attempt to close the HeadGitRepo.

Fix #17193

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

* add TEST

Co-authored-by: 6543 <6543@obermui.de>

integrations/compare_test.go

@@ -6,6 +6,7 @@ package integrations
 
 import (
 	"net/http"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -21,4 +22,8 @@ func TestCompareTag(t *testing.T) {
 	selection := htmlDoc.doc.Find(".choose.branch .filter.dropdown")
 	// A dropdown for both base and head.
 	assert.Lenf(t, selection.Nodes, 2, "The template has changed")
+
+	req = NewRequest(t, "GET", "/user2/repo1/compare/invalid")
+	resp = session.MakeRequest(t, req, http.StatusNotFound)
+	assert.False(t, strings.Contains(resp.Body.String(), "/assets/img/500.png"), "expect 404 page not 500")
 }