cmake: do not optimize away nullptr checks

OBJECTTYPE::exists() and SOLDIERTYPE::exists()/DeleteSoldier() are
called on null pointers by design and guard with `this == NULL`. That
is undefined behaviour, so clang infers `this` is non-null: it deletes
the guard inside the callee *and* deletes null checks that follow a
call in the caller, which is an access violation at /O2 in code MSVC
has always compiled the naive way.  The caller-side inference happens
in every translation unit that calls one of these, so this has to be
global rather than per-file; there is no per-function attribute or
pragma for it. Drop it once nothing relies on a null `this`.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -42,6 +42,10 @@ if(ADDRESS_SANITIZER)
   add_compile_options($<IF:$<OR:$<CONFIG:Debug>,$<CONFIG:RelWithDebInfo>>,-fsanitize=address,>)
 endif()
 
+if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  add_compile_options(-fno-delete-null-pointer-checks)
+endif()
+
 # whether we are using MSBuild as a generator
 set(usingMsBuild $<STREQUAL:${CMAKE_VS_PLATFORM_NAME},Win32>)