check if player is null when updating card menu

[ebbit1q]

this should fix this crash:

#0  0x000055c5bb6be035 in Player::updateCardMenu (this=0x0, card=0x55c5bf712170)
    at Cockatrice/cockatrice/src/player.cpp:3308
        cardMenu = 0x55c5bb6cb550 <QMapData<int, Player*>::findNode(int const&) const+58>
        ptMenu = 0x55c5bf96c45c
        moveMenu = 0x55c5bdb0cd00
        revealedCard = false
        writeableCard = false
#1  0x000055c5bb75850b in TabGame::updateCardMenu (this=0x55c5bf96c380, card=0x55c5bf712170)
    at Cockatrice/cockatrice/src/tab_game.cpp:1420
        p = 0x0
#2  0x000055c5bb622001 in TabGame::qt_static_metacall (_o=0x55c5bf96c380, 
    _c=QMetaObject::InvokeMetaMethod, _id=16, _a=0x7ffce85e8a20)
    at Cockatrice/build/cockatrice/cockatrice_autogen/UVLADIE3JM/moc_tab_game.cpp:558
        _t = 0x55c5bf96c380
        ```