item::gun_range dereferences null pointer

[ymber]

Describe the bug

The game crashes when item::gun_range tries to dereference a null pointer. When that method is called from an object that contains no ammo items and has a gunmod that defines ammo_modifier it will end up trying to dereference the ammo islot of the null item.

Steps To Reproduce

1. Debug in one of the old caliber conversion kits and a gun
2. Install the conversion kit on the gun
3. Go to your character's inventory and select the gun

Expected behavior

Normal inventory function without crashes.

Versions and configuration

• OS: Arch Linux x86_64 kernel 5.1.16-arch1-1-ARCH
• Game Version: 0.D-5504-g4f3fee78cf

Additional context

The program has crashed.
See the log file for a stack trace.
CRASH LOG FILE: ./config/crash.log
VERSION: 0.D-5504-g4f3fee78cf
TYPE: Signal
MESSAGE: SIGABRT: Abnormal termination
STACK TRACE:

	./cataclysm-tiles(_Z21debug_write_backtraceRSo+0x39) [0x55c98f587059]
	./cataclysm-tiles(+0x133e0b3) [0x55c98f5690b3]
	./cataclysm-tiles(+0x133de58) [0x55c98f568e58]
	/usr/lib/libc.so.6(+0x3a7e0) [0x7f8bb71407e0]
	/usr/lib/libc.so.6(gsignal+0x145) [0x7f8bb7140755]
	/usr/lib/libc.so.6(abort+0x125) [0x7f8bb712b851]
	/usr/lib/libc.so.6(+0x25727) [0x7f8bb712b727]
	/usr/lib/libc.so.6(+0x33026) [0x7f8bb7139026]
	./cataclysm-tiles(+0x11b4f0d) [0x55c98f3dff0d]
	./cataclysm-tiles(_ZNK4cata8optionalI10islot_ammoEptEv+0x6) [0x55c98f417096]
	./cataclysm-tiles(_ZNK4item9gun_rangeEb+0x104) [0x55c98f7f9854]
	./cataclysm-tiles(_ZNK4item9gun_rangeEPK6player+0x2c) [0x55c98f7ed40c]
	./cataclysm-tiles(_ZNK4item4infoB5cxx11ERSt6vectorI8iteminfoSaIS1_EEPK14iteminfo_queryi+0x48f9) [0x55c98f7d8bf9]
	./cataclysm-tiles(_ZNK4item4infoB5cxx11EbRSt6vectorI8iteminfoSaIS1_EEi+0x26) [0x55c98f7d42f6]
	./cataclysm-tiles(_ZNK4item4infoB5cxx11EbRSt6vectorI8iteminfoSaIS1_EE+0xf) [0x55c98f7d42bf]
	./cataclysm-tiles(_ZN4game19inventory_item_menuEiiiNS_27inventory_item_menu_positonE+0x8af) [0x55c98f69443f]
	./cataclysm-tiles(_ZN10game_menus3inv6commonER6avatar+0x1b5) [0x55c98f70d235]
	./cataclysm-tiles(_ZN4game13handle_actionEv+0xf04) [0x55c98f72e6f4]
	./cataclysm-tiles(_ZN4game7do_turnEv+0x52d) [0x55c98f68bb5d]
	./cataclysm-tiles(main+0xbc0) [0x55c98f984150]

	Attempting to repeat stack trace using debug symbols...
	debug_write_backtrace(std::ostream&)
	.../src/debug.cpp:620
	log_crash(char const*, char const*)
	crash.cpp:?
	signal_handler(int)
	.../src/crash.cpp:292
	__restore_rt
	sigaction.c:?
	__GI_raise
	:?
	__GI_abort
	:?
	_nl_load_domain.cold
	loadmsgcat.c:?
	__GI___assert_fail
	:?
	cata::optional<islot_ammo>::get() const
	??:?
	cata::optional<islot_ammo>::operator->() const
	.../src/optional.h:105
	item::gun_range(bool) const
	.../src/item.cpp:5770
	item::gun_range(player const*) const
	.../src/item.cpp:5777
	item::info[abi:cxx11](std::vector<iteminfo, std::allocator<iteminfo> >&, iteminfo_query const*, int) const
	.../src/item.cpp:1501
	item::info[abi:cxx11](bool, std::vector<iteminfo, std::allocator<iteminfo> >&, int) const
	.../src/item.cpp:876
	item::info[abi:cxx11](bool, std::vector<iteminfo, std::allocator<iteminfo> >&) const
	.../src/item.cpp:871
	game::inventory_item_menu(int, int, int, game::inventory_item_menu_positon)
	??:?
	game_menus::inv::common(avatar&)
	.../src/game_inventory.cpp:202
	game::handle_action()
	.../src/handle_action.cpp:1795
	game::do_turn()
	.../src/game.cpp:1428
	main
	.../src/main.cpp:688