Fix reference binding to null

/usr/include/c++/10.2.0/bits/stl_vector.h:1046:34: runtime error: reference binding to null pointer of type 'value_type'
    #0 0x55e37f50008a in std::vector<char, std::allocator<char> >::operator[](unsigned long) /usr/include/c++/10.2.0/bits/stl_vector.h:1046
    #1 0x55e37f50008a in ESM::SavedGame::load(ESM::ESMReader&) /home/elsid/dev/openmw/components/esm/savedgame.cpp:28
    #2 0x55e37e726139 in MWState::Character::addSlot(boost::filesystem::path const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/apps/openmw/mwstate/character.cpp:31
    #3 0x55e37e742b39 in MWState::Character::Character(boost::filesystem::path const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/apps/openmw/mwstate/character.cpp:88
    #4 0x55e37e7006e1 in MWState::CharacterManager::CharacterManager(boost::filesystem::path const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/apps/openmw/mwstate/charactermanager.cpp:25
    #5 0x55e37e6d4140 in MWState::StateManager::StateManager(boost::filesystem::path const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/apps/openmw/mwstate/statemanagerimp.cpp:90
    #6 0x55e37e82595a in OMW::Engine::prepareEngine(Settings::Manager&) /home/elsid/dev/openmw/apps/openmw/engine.cpp:641
    #7 0x55e37e8439fd in OMW::Engine::go() /home/elsid/dev/openmw/apps/openmw/engine.cpp:867
    #8 0x55e37e782760 in runApplication(int, char**) /home/elsid/dev/openmw/apps/openmw/main.cpp:289
    #9 0x55e37f6483c3 in wrapApplication(int (*)(int, char**), int, char**, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /home/elsid/dev/openmw/components/debug/debugging.cpp:200
    #10 0x55e37ba8e3fe in main /home/elsid/dev/openmw/apps/openmw/main.cpp:301
    #11 0x7f013e845151 in __libc_start_main (/usr/lib/libc.so.6+0x28151)
    #12 0x55e37baa0e3d in _start (/home/elsid/dev/openmw/build/gcc/ubsan/openmw+0x6c11e3d)

components/esm/savedgame.cpp

@@ -25,7 +25,7 @@ void ESM::SavedGame::load (ESMReader &esm)
     esm.getSubNameIs("SCRN");
     esm.getSubHeader();
     mScreenshot.resize(esm.getSubSize());
-    esm.getExact(&mScreenshot[0], mScreenshot.size());
+    esm.getExact(mScreenshot.data(), mScreenshot.size());
 }
 
 void ESM::SavedGame::save (ESMWriter &esm) const