filesystem: fix crashes caused by returning a pointer from an std::vector (#4043)

Co-authored-by: Stephen Miller <56742918+StevenMiller123@users.noreply.github.com>

src/core/file_sys/fs.cpp

@@ -55,7 +55,7 @@ std::filesystem::path MntPoints::GetHostPath(std::string_view path, bool* is_rea
     if (path.length() > 255)
         return "";
 
-    const MntPair* mount = GetMount(corrected_path);
+    const std::optional<MntPair> mount = GetMount(corrected_path);
     if (!mount) {
         return "";
     }

src/core/file_sys/fs.h

@@ -5,6 +5,7 @@
 
 #include <atomic>
 #include <mutex>
+#include <optional>
 #include <string>
 #include <vector>
 #include <tsl/robin_map.h>
@@ -58,14 +59,17 @@ public:
         return it == m_mnt_pairs.end() ? nullptr : &*it;
     }
 
-    const MntPair* GetMount(const std::string& guest_path) {
+    const std::optional<MntPair> GetMount(const std::string& guest_path) {
         std::scoped_lock lock{m_mutex};
         const auto it = std::ranges::find_if(m_mnt_pairs, [&](const auto& mount) {
             // When doing starts-with check, add a trailing slash to make sure we don't match
             // against only part of the mount path.
             return guest_path == mount.mount || guest_path.starts_with(mount.mount + "/");
         });
-        return it == m_mnt_pairs.end() ? nullptr : &*it;
+        if (it == m_mnt_pairs.end()) {
+            return std::nullopt;
+        }
+        return *it;
     }
 
 private:

src/core/libraries/save_data/save_instance.cpp

@@ -105,7 +105,7 @@ SaveInstance::SaveInstance(int slot_num, Libraries::UserService::OrbisUserServic
     mount_point = "/savedata" + std::to_string(slot_num);
 
     this->exists = fs::exists(param_sfo_path);
-    this->mounted = g_mnt->GetMount(mount_point) != nullptr;
+    this->mounted = g_mnt->GetMount(mount_point) != std::nullopt;
 }
 
 SaveInstance::~SaveInstance() {