fix(session): guard null active session in xwayland primary output sync

[glyvut]

Check the weak_ptr lock result before dereferencing the active session. When an output is removed, primaryOutputChanged is delivered synchronously; if the active session was already reset (e.g. after a logind session was removed), calling ->xwayland() on the null session crashes the compositor.

输出移除时同步触发primaryOutputChanged，若活动会话已被重置，
对空指针调用->xwayland()会导致合成器崩溃，增加空指针守卫。

Log: 修复输出移除时活动会话为空导致的崩溃

Summary by Sourcery

Bug Fixes:

• Prevent compositor crashes when the active session is unavailable during Xwayland primary-output synchronization.

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Guard against a null active session in XWayland primary-output sync to prevent compositor crashes when outputs are removed and the active session has already been reset.

Sequence diagram for guarded XWayland primary-output sync

sequenceDiagram
    participant Compositor
    participant SessionManager
    participant ActiveSession
    participant XWayland

    Compositor->>SessionManager: syncActiveSessionXWaylandPrimaryOutput()
    SessionManager->>SessionManager: primaryOutput check
    SessionManager->>ActiveSession: m_activeSession.lock()
    alt session is null
        SessionManager-->>Compositor: return (skip XWayland sync)
    else session is valid
        SessionManager->>XWayland: session.xwayland()
        XWayland->>XWayland: xcbConnection()
        XWayland->>XWayland: xcbScreen()
        SessionManager-->>Compositor: XWayland primary output synced
    end

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File-Level Changes

Change	Details	Files
Add a null-check for the locked active session before using it in XWayland primary-output synchronization to prevent crashes.	Lock m_activeSession into a local variable before use instead of calling activeSession().lock() inline Return early if the locked active session is null Use the locally locked session pointer to access xwayland() instead of dereferencing a potentially null session	src/session/session.cpp

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: glyvut, zccrs

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