Fix assignment to cm.AssigneeID when importing comments (#22528)

This is a fix for https://github.com/go-gitea/gitea/pull/22510

The code assumed that the `AssigneeID` from the comment YAML was an
`int64`, but it is actually an `int`, causing a panic. It also had no
check on whether the type cast was actually valid, so badly formatted
YAML could also cause a panic.

Both these issues have been fixed.

services/migrations/gitea_uploader.go

@@ -468,7 +468,9 @@ func (g *GiteaLocalUploader) CreateComments(comments ...*base.Comment) error {
 
 		switch cm.Type {
 		case issues_model.CommentTypeAssignees:
-			cm.AssigneeID = comment.Meta["AssigneeID"].(int64)
+			if assigneeID, ok := comment.Meta["AssigneeID"].(int); ok {
+				cm.AssigneeID = int64(assigneeID)
+			}
 			if comment.Meta["RemovedAssigneeID"] != nil {
 				cm.RemovedAssignee = true
 			}