Achievements: Fix leaderboard screen use-after-free on close

rc_client frees a request's async handle right after calling back. The
error path didn't clear pendingAsyncCall_, so closing the screen after a
failed fetch aborted a freed handle. Switching between Top players and
Around me also overwrote the handle without aborting the old request,
which could then call back into a closed screen, or leak an entry list.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

UI/RetroAchievementScreens.cpp

@@ -258,20 +258,28 @@ RetroAchievementsLeaderboardScreen::RetroAchievementsLeaderboardScreen(const Pat
 
 void RetroAchievementsLeaderboardScreen::FetchEntries() {
 	auto callback = [](int result, const char *error_message, rc_client_leaderboard_entry_list_t *list, rc_client_t *client, void *userdata) {
+		RetroAchievementsLeaderboardScreen *thiz = (RetroAchievementsLeaderboardScreen *)userdata;
+		// rc_client frees the handle after this returns, so it must not be aborted later.
+		thiz->pendingAsyncCall_ = nullptr;
 		if (result != RC_OK) {
 			g_OSD.Show(OSDType::MESSAGE_ERROR, error_message, 10.0f);
 			return;
 		}
-
-		RetroAchievementsLeaderboardScreen *thiz = (RetroAchievementsLeaderboardScreen *)userdata;
+		if (thiz->pendingEntryList_) {
+			rc_client_destroy_leaderboard_entry_list(thiz->pendingEntryList_);
+		}
 		thiz->pendingEntryList_ = list;
-		thiz->pendingAsyncCall_ = nullptr;
 	};
 
+	// An aborted request doesn't call back, so only the latest request can reach the callback.
+	if (pendingAsyncCall_) {
+		rc_client_abort_async(Achievements::GetClient(), pendingAsyncCall_);
+		pendingAsyncCall_ = nullptr;
+	}
+
 	// Store the handle so the destructor can abort it if we're closed before the
 	// response arrives - without this, the callback above would run later against
-	// a freed `this` (its pendingAsyncCall_ null-check in the destructor was
-	// otherwise always a no-op since this was never assigned).
+	// a freed `this`.
 	if (nearMe_) {
 		pendingAsyncCall_ = rc_client_begin_fetch_leaderboard_entries_around_user(Achievements::GetClient(), leaderboardID_, 10, callback, this);
 	} else {