TransferSizeMismatchException can be raised even if remote size is not reported

[jackBonadies]

SoulseekQt can report a size of 0 if file is larger than 2GB, so only throw for valid (>0) remote size. Nicotine+ has this same guard in place, and I think it makes sense (if FileSize is <=0 then it means the remote client did not report it therefore it is not true that the file size has "changed")

Sample fix