protocols: Fix IWaylandProtocol onDisplayDestroy m_pGlobal double-free

[PlasmaPower]

Describe your PR, what does it fix/add?

This fixes a segfault on Hyprland shutdown caused by the wl_global_destroy call modified in this PR:

#0  0x00007a75f51443af in wl_list_remove (elm=elm@entry=0x5ec7ca5e15a8) at ../wayland-1.23.1/src/wayland-util.c:56
#1  0x00007a75f5145cb0 in wl_global_destroy (global=0x5ec7ca5e1580) at ../wayland-1.23.1/src/wayland-server.c:1399
#2  0x00005ec78c6d7017 in IWaylandProtocol::onDisplayDestroy (this=0x5ec7ca5e1620)
    at /home/lee/code/cpp/Hyprland/src/protocols/WaylandProtocol.cpp:17
#3  0x00005ec78c6d7707 in IWaylandProtocol::~IWaylandProtocol (this=0x5ec7ca5e1620)
    at /home/lee/code/cpp/Hyprland/src/protocols/WaylandProtocol.cpp:37

Looking at the code, m_pGlobal will be destroyed first when the display is destroyed by the onDisplayDestroy hook, and then a second time when the wayland protocol is destroyed (if that happens later) by the IWaylandProtocol destructor calling onDisplayDestroy a second time. This PR fixes that by setting m_pGlobal to nullptr after destroying it and adding null checks.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

I don't think the m_pGlobal null check in removeGlobal is necessary, but better safe than sorry I figure.

The m_liDisplayDestroy.listener.link removal is fine as-is because it's already re-init'ed afterwards in the existing code, meaning that the second removal will be a no-op.

Is it ready for merging, or does it need work?

Ready for merging

[vaxerski]

luv me sum c, thanks!