download file content is not valid if use ChunkSize is bigger than the file content size

[simonwu-os]

I use --chunk 5242880 to specify ChunkSize.
If the size of the target file is less then 5M, the content downloaded is not valid. Its sha256 is not right.
I check the source code, I find it has a bug. When d.info.Size is less than d.ChunkSize, then chunksLen is zero.

I fix it in my fork https://github.com/simonwu-os/got.
It works fine.

chunksLen := d.info.Size / d.ChunkSize
///added by simon wu
///fix if chunksLen = 0, then the result file content is not valid.
if chunksLen == 0 {
chunksLen += 1
}
///end of added