xts_archive: Check if the file is nullptr prior to parsing

Fixes an access violation where the file no longer exists at the specified path while being parsed.

src/core/file_sys/xts_archive.cpp

@@ -70,14 +70,18 @@ NAX::NAX(VirtualFile file_, std::array<u8, 0x10> nca_id)
 NAX::~NAX() = default;
 
 Loader::ResultStatus NAX::Parse(std::string_view path) {
-    if (file->ReadObject(header.get()) != sizeof(NAXHeader))
+    if (file == nullptr) {
+        return Loader::ResultStatus::ErrorNullFile;
+    }
+    if (file->ReadObject(header.get()) != sizeof(NAXHeader)) {
         return Loader::ResultStatus::ErrorBadNAXHeader;
-
-    if (header->magic != Common::MakeMagic('N', 'A', 'X', '0'))
+    }
+    if (header->magic != Common::MakeMagic('N', 'A', 'X', '0')) {
         return Loader::ResultStatus::ErrorBadNAXHeader;
-
-    if (file->GetSize() < NAX_HEADER_PADDING_SIZE + header->file_size)
+    }
+    if (file->GetSize() < NAX_HEADER_PADDING_SIZE + header->file_size) {
         return Loader::ResultStatus::ErrorIncorrectNAXFileSize;
+    }
 
     keys.DeriveSDSeedLazy();
     std::array<Core::Crypto::Key256, 2> sd_keys{};