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>

routers/web/repo/compare.go

@@ -635,7 +635,7 @@ func getBranchesAndTagsForRepo(user *models.User, repo *models.Repository) (bool
 func CompareDiff(ctx *context.Context) {
 	ci := ParseCompareInfo(ctx)
 	defer func() {
-		if ci.HeadGitRepo != nil {
+		if ci != nil && ci.HeadGitRepo != nil {
 			ci.HeadGitRepo.Close()
 		}
 	}()

routers/web/repo/pull.go

@@ -1043,7 +1043,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 
 	ci := ParseCompareInfo(ctx)
 	defer func() {
-		if ci.HeadGitRepo != nil {
+		if ci != nil && ci.HeadGitRepo != nil {
 			ci.HeadGitRepo.Close()
 		}
 	}()