If the slave didn't pass a modification time (e.g. the http slave doesn't), it was set to -1,
resulting in setting a wrong modification time for the destination file in copyNextFile() later on because that
case wasn't checked.
So only set info.mtime when the slave actually provided a value.
There's no need for further checks later in copyNextFile() (where FileCopyJob::setModificationTime() is called)
because FileCopyJob checks for validity anyway.
BUG: 374420