SI_DeviceGBA: use SI_ERROR_NO_RESPONSE when client isn't connected

Slight behavior change, but fills a gaping hole in the state logic.

Source/Core/Core/HW/SI/SI_DeviceGBA.cpp

@@ -5,6 +5,7 @@
 #include "Core/HW/SI/SI_DeviceGBA.h"
 
 #include <cstddef>
+#include <cstring>
 #include <memory>
 #include <mutex>
 #include <queue>
@@ -303,6 +304,12 @@ int CSIDevice_GBA::RunBuffer(u8* buffer, int length)
 #endif
       m_sock_server.Send(buffer);
     }
+    else
+    {
+      constexpr u32 reply = SI_ERROR_NO_RESPONSE;
+      std::memcpy(buffer, &reply, sizeof(reply));
+      return sizeof(reply);
+    }
     m_last_cmd = buffer[3];
     m_timestamp_sent = CoreTiming::GetTicks();
     m_state = ConnectionState::WaitForTransferTime;