fix: dont crash when equipping npc with no existing clothing

[ekaratzas]

Purpose of change

• fixes Equipping items onto an NPC from the armor sort menu segfaults if item is on floor on same tile as player. #2743
• fixes Equipping NPCs equipment crashes the game. #4353

Fix crash when equipping armor to npc with no clothing.

Describe the solution

Crash due to accessing index 0 of empty std::vector. Skip accessing that vector in that corner-case.

Relevant backtrace

#0  operator() (__closure=0x7fffffffc4a0, index=0) at /home/gpf/gamedev/Cataclysm-BN/src/armor_layers.cpp:498
#1  0x00005555557180c9 in show_armor_layers_ui (who=...) at /home/gpf/gamedev/Cataclysm-BN/src/armor_layers.cpp:889
#2  0x000055555661c1b7 in game::npc_menu (this=this@entry=0x555558b3c190, who=...) at /home/gpf/gamedev/Cataclysm-BN/src/game.cpp:5383
#3  0x000055555661cded in game::examine (this=this@entry=0x555558b3c190, examp=...) at /home/gpf/gamedev/Cataclysm-BN/src/game.cpp:5534
#4  0x000055555661e0f3 in game::examine (this=this@entry=0x555558b3c190) at /usr/include/c++/11/optional:272
#5  0x00005555566ac11a in game::handle_action (this=this@entry=0x555558b3c190) at /home/gpf/gamedev/Cataclysm-BN/src/handle_action.cpp:1911
#6  0x0000555556622605 in game::do_turn (this=0x555558b3c190) at /home/gpf/gamedev/Cataclysm-BN/src/game.cpp:1450
#7  0x00005555555c9c84 in main (argc=0, argv=0x7fffffffe050) at /usr/include/c++/11/bits/unique_ptr.h:173

Describe alternatives you've considered

dda has a vastly different/rewritten? framework here.

Testing

take off all clothes/armor from npc.
try to equip armor from floor via sort armor menu.
no crash.
npc actually equips armor ok.

Additional context

Yes my compiler complained without that weird casting around nullopt.

Checklist

[KheirFerrum]

• Check if related to Equipping items onto an NPC from the armor sort menu segfaults if item is on floor on same tile as player. #2743

[scarf005]

2024-03-15_16-02-09.mp4