Fix opening a write-only file, beiung careful not to destroy its contents.
See http://lists.squeakfoundation.org/pipermail/vm-dev/2014-June/015699.html


git-svn-id: http://squeakvm.org/svn/squeak/trunk/platforms/Cross/plugins@2949 fa1542d4-bde8-0310-ad64-8ed1123d492a