MogileFS::Util: avoid calling syswrite on zero-length buffer
MogileFS::Util: avoid calling syswrite on zero-length buffer

This bug can't have a noticeable effect since writing a
zero-length buffer would result in nothing being written, but
this is more correct and can save a method dispatch and syscall.

godfat/not-used-mogilefs-client,songkick/mogilefs-client,drasch/mogilefs-client