storage: only run safezero if allocation is > 0
    
    While a zero allocation in safezero should be fine it isn't when we use
    posix_fallocate which returns EINVAL on a zero allocation.
    
    While we could skip the zero allocation in safezero_posix_fallocate it's
    an optimization to do it for all allocations.
    
    This fixes vm installation via virtinst for me which otherwise aborts
    like:
    
       Starting install...
       Retrieving file linux...               | 5.9 MB     00:01 ...
       Retrieving file initrd.gz...           |  29 MB     00:07 ...
       ERROR    Couldn't create storage volume 'virtinst-linux.sBgds4': 'cannot fill file '/var/lib/libvirt/boot/virtinst-linux.sBgds4': Invalid argument'
    
    The error was introduced by e30297b0 as spotted by Chunyan Liu