Fix stange behavior of DownloadPullDiffOrPatch in incorect index (#17223) (#17227)

Fix GetPullRequestByIndex by validate index > 1

Signed-off-by: Danila Kryukov <pricly_yellow@dismail.de>
Co-authored-by: a1012112796 <1012112796@qq.com>

models/pull.go

@@ -502,6 +502,9 @@ func GetLatestPullRequestByHeadInfo(repoID int64, branch string) (*PullRequest,
 
 // GetPullRequestByIndex returns a pull request by the given index
 func GetPullRequestByIndex(repoID, index int64) (*PullRequest, error) {
+	if index < 1 {
+		return nil, ErrPullRequestNotExist{}
+	}
 	pr := &PullRequest{
 		BaseRepoID: repoID,
 		Index:      index,