[Memory] Close shared memory FD and properly handle its invalid value

src/xenia/cpu/backend/x64/x64_code_cache.cc

@@ -40,11 +40,11 @@ X64CodeCache::~X64CodeCache() {
   }
 
   // Unmap all views and close mapping.
-  if (mapping_) {
+  if (mapping_ != xe::memory::kFileMappingHandleInvalid) {
     xe::memory::UnmapFileView(mapping_, generated_code_base_,
                               kGeneratedCodeSize);
     xe::memory::CloseFileMappingHandle(mapping_, file_name_);
-    mapping_ = nullptr;
+    mapping_ = xe::memory::kFileMappingHandleInvalid;
   }
 }
 
@@ -67,7 +67,7 @@ bool X64CodeCache::Initialize() {
   mapping_ = xe::memory::CreateFileMappingHandle(
       file_name_, kGeneratedCodeSize, xe::memory::PageAccess::kExecuteReadWrite,
       false);
-  if (!mapping_) {
+  if (mapping_ == xe::memory::kFileMappingHandleInvalid) {
     XELOGE("Unable to create code cache mmap");
     return false;
   }

src/xenia/cpu/backend/x64/x64_code_cache.h

@@ -100,7 +100,8 @@ class X64CodeCache : public CodeCache {
                          UnwindReservation unwind_reservation) {}
 
   std::filesystem::path file_name_;
-  xe::memory::FileMappingHandle mapping_ = nullptr;
+  xe::memory::FileMappingHandle mapping_ =
+      xe::memory::kFileMappingHandleInvalid;
 
   // NOTE: the global critical region must be held when manipulating the offsets
   // or counts of anything, to keep the tables consistent and ordered.