Add thread check to AchievementManager MemoryPeeker

rc_client calls the provided memory peeker asynchronously in the callback for starting a session, to validate/invalidate the memory used for achievements. Dolphin cannot access memory from any thread but host or CPU so this access has a small chance of being invalid. This adds a verification to that method to ensure that memory is only being accessed on the correct threads but that the response when this isn't the case is what rc_client expects.

Source/Core/Core/AchievementManager.cpp

@@ -22,6 +22,7 @@
 #include "Common/WorkQueueThread.h"
 #include "Core/Config/AchievementSettings.h"
 #include "Core/Core.h"
+#include "Core/HW/Memmap.h"
 #include "Core/PowerPC/MMU.h"
 #include "Core/System.h"
 #include "DiscIO/Blob.h"
@@ -919,6 +920,16 @@ u32 AchievementManager::MemoryPeeker(u32 address, u8* buffer, u32 num_bytes, rc_
   if (buffer == nullptr)
     return 0u;
   auto& system = Core::System::GetInstance();
+  // If this is called in some other thread, for example a callback, we want to make sure we return
+  // how many bytes we *could* read even if we can't, because rc_client will deactivate achievements
+  // and leaderboards as "unsupported" if the memory access is invalid.
+  if (!(Core::IsHostThread() || Core::IsCPUThread()))
+  {
+    u32 ram_size = system.GetMemory().GetRamSizeReal();
+    if (address >= ram_size)
+      return 0;
+    return std::min(ram_size - address, num_bytes);
+  }
   Core::CPUThreadGuard threadguard(system);
   for (u32 num_read = 0; num_read < num_bytes; num_read++)
   {