fix: avoid index out-of-range panic in webhook assignee formatting (#39827)

getPullRequestInfo and getIssuesInfo indexed into the last element of an
empty Assignees slice when Action was HookIssueAssigned, causing a panic
under a race where the assignee list is empty by the time the webhook
payload is built.

Fixes https://github.com/go-gitea/gitea/issues/39826
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

modules/structs/hook.go

@@ -393,7 +393,7 @@ type IssuePayload struct {
 	Action HookIssueAction `json:"action"`
 	// The index number of the issue
 	Index int64 `json:"number"`
-	// Changes made to the issue (for edit actions)
+	// Changes made to the issue
 	Changes *ChangesPayload `json:"changes,omitempty"`
 	// The issue that was acted upon
 	Issue *Issue `json:"issue"`
@@ -430,6 +430,10 @@ type ChangesPayload struct {
 	AddedLabels []*Label `json:"added_labels"`
 	// Changes made to the labels removed
 	RemovedLabels []*Label `json:"removed_labels"`
+	// Assignees added
+	AddedAssignees []*User `json:"added_assignees,omitempty"`
+	// Assignees removed
+	RemovedAssignees []*User `json:"removed_assignees,omitempty"`
 }
 
 // PullRequestPayload represents a payload information of pull request event.
@@ -442,7 +446,7 @@ type PullRequestPayload struct {
 	After string `json:"after,omitempty"`
 	// The index number of the pull request
 	Index int64 `json:"number"`
-	// Changes made to the pull request (for edit actions)
+	// Changes made to the pull request
 	Changes *ChangesPayload `json:"changes,omitempty"`
 	// The pull request that was acted upon
 	PullRequest *PullRequest `json:"pull_request"`