Segfault on map open or after few moves

[Xelat84]

Describe the bug
Game just crash when I open map (press 'm'), or make few moves.

VERSION: 0.D-1679-gdd4532a
TYPE: Signal
MESSAGE: SIGSEGV: Segmentation fault
STACK TRACE:
    @0x5583A5[cataclysm-tiles.exe+0x1583A5]
    @0x558F52[cataclysm-tiles.exe+0x158F52]
    SMPEG_error+0x4B034@0xE3F030[cataclysm-tiles.exe+0xA3F030]
    _C_specific_handler+0x9C@0x77BB85A8[ntdll.dll+0x185A8]
    RtlDecodePointer+0xBD@0x77BC9D0D[ntdll.dll+0x29D0D]
    RtlUnwindEx+0xBBF@0x77BB91AF[ntdll.dll+0x191AF]
    KiUserExceptionDispatcher+0x2E@0x77BF1278[ntdll.dll+0x51278]
    @0xAF2221[cataclysm-tiles.exe+0x6F2221]
    @0xAF7CCD[cataclysm-tiles.exe+0x6F7CCD]
    @0xAFF2FB[cataclysm-tiles.exe+0x6FF2FB]
    @0x69C468[cataclysm-tiles.exe+0x29C468]
    @0x6800C7[cataclysm-tiles.exe+0x2800C7]
    IMG_LoadWEBP_RW+0x4E16C9@0x1412569[cataclysm-tiles.exe+0x1012569]
    @0x4013ED[cataclysm-tiles.exe+0x13ED]
    @0x4014FB[cataclysm-tiles.exe+0x14FB]
    BaseThreadInitThunk+0xD@0x77A9652D[kernel32.dll+0x1652D]
    RtlUserThreadStart+0x21@0x77BCC521[ntdll.dll+0x2C521]

To Reproduce
I provide my savegame and config folder, so it's enough to load that savegame and press 'm'.

Versions and configuration(please complete the following information):

• OS: Windows 7 x64
• Game Version: 0.D-1679-gdd4532a
• Graphics version: Tiles
• Mods loaded: [ "dda", "no_npc_food", "novitamins", "More_Survival_Tools", "mutant_npcs", "national_guard_camp", "more_locations", "Urban_Development", "boats", "blazemod", "Tanks", "no_olg_guns", "no_religious_Texts", "no_reviving_zombies", "realguns", "growable-pots", "modular_turrets", "Salvaged_Robots", "FujiStruct" ]

Additional context
Link to a savegame: https://yadi.sk/d/gJvEHkncdmGOMg

[neitsa]

Thanks @Xelat84 for your bug report! The crash can be easily reproduced with your save file.

Stack trace (lines may not be totally accurate, my build is some PRs behind master):

>	Cataclysm.exe!`anonymous namespace'::draw(const catacurses::window & w, const catacurses::window & wbar, const tripoint & center, const tripoint & orig, bool blink, bool show_explored, bool fast_scroll, input_context * inp_ctxt, const `anonymous-namespace'::draw_data_t & data) Line 513	C++
 	Cataclysm.exe!`anonymous namespace'::display(const tripoint & orig, const `anonymous-namespace'::draw_data_t & data) Line 968	C++
 	Cataclysm.exe!ui::omap::display() Line 1330	C++
 	Cataclysm.exe!game::handle_action() Line 1805	C++
 	Cataclysm.exe!game::do_turn() Line 1401	C++
 	Cataclysm.exe!SDL_main(int argc, char * * argv) Line 682	C++
 	Cataclysm.exe!main_getcmdline(...) Line 177	C
 	[External Code]	

In overmap_ui.cpp; draw function; line 506:

        std::vector<npc *> followers;
        for( auto &elem : g->get_follower_list() ) {
            std::shared_ptr<npc> npc_to_get = overmap_buffer.find_npc( elem );
            npc *npc_to_add = npc_to_get.get(); // returns nullptr
            followers.push_back( npc_to_add ); // push nullptr
        }
        for( const auto &np : followers ) {
            if( np->posz() != z ) {      // **crash** here because np = nullptr
                continue;
            }

It seems that npc *npc_to_add = npc_to_get.get(); returns null pointers, so followers is filled with one or multiple nullptr and the next range-based for loop iterates on nullptr, giving a crash on np->posz().

>? g->get_follower_list()
{ size=2 }
    [capacity]: 2
    [allocator]: allocator
    [0]: 90
    [1]: 28
    [Raw View]: {...}

> ? followers
{ size=2 }
    [capacity]: 2
    [allocator]: allocator
    [0]: 0x0000000000000000 <NULL>
    [1]: 0x0000000000000000 <NULL>
    [Raw View]: {...}