Another issue from #334 with the casting moved from the output of
min to it's inputs.
This is a non-issue on 64 bit machines, but if dolphin is compiled
on an OS with size_t == u32 (say ARM) then remainingSize could be
truncated.
Restored the casting to the original order before #334.
When CFileSystemGCWii::GetFileName can't find a valid filename it would return nullptr.
nullptr as a std::string throws an assert within the std lib.
So return an empty string and check if it is empty or not
