When moving a file over device boundaries and unlinking the source file fails

because it does not exist anymore, don't unlink the destination file.

While it is unclear to me how/when this can happen, at least one user
encountered the problem:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=273476


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@834697 13f79535-47bb-0310-9956-ffa450edef68