InputCommon: improve code that returns a controller attachment index

casting a value to a u32 when it's originally an int, and it's exposed as int to users,
could end up in cases where a negative number would result as a positive one.
This doesn't really affect the value range of the attachment enum,
still I think the code was wrong.

Heavily tested.

Source/Core/InputCommon/ControllerEmu/ControlGroup/Attachments.cpp

@@ -17,10 +17,11 @@ void Attachments::AddAttachment(std::unique_ptr<EmulatedController> att)
 
 u32 Attachments::GetSelectedAttachment() const
 {
-  const u32 value = m_selection_value.GetValue();
+  // This is originally an int, treat it as such
+  const int value = m_selection_value.GetValue();
 
-  if (value < m_attachments.size())
-    return value;
+  if (value > 0 && static_cast<size_t>(value) < m_attachments.size())
+    return u32(value);
 
   return 0;
 }