Fix build failure in linux

[csknns]

Fixes #277

It seems that Swift is unwilling to implicitly unwrap the optional filePointer: UnsafeMutablePointer? when passing it to fwrite in linux, you can see this build failure in building a dependand library here.

So I unwrap the filePointer before writing bytes. Also I improved the while loop that reads bytes from the zip file until there are no more, the condition before was while filePointer != nil that was always true, not the condition is that read method returns that zero bytes where read.