virStrncpy: fix to successfully copy empty string
virStrncpy: fix to successfully copy empty string

After [1] we got failure on attempt to copy empty string.
Before the patch empty string was copied successfuly.
Restore the original behaviour.

[1] 7d70a63b util: Improve virStrncpy() implementation

Signed-off-by: Nikolay Shirokovskiy <7fa0e2698a05be944b362bdf5d7736c0162e16c9@virtuozzo.com>
Reviewed-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>

andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,andreabolognani/libvirt,crobinso/libvirt,zippy2/libvirt,fabianfreyer/libvirt,zippy2/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,jardasgit/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,jardasgit/libvirt,jardasgit/libvirt