Don't try to mmap the contents of empty files. This behaviour was harmless
prior to r195693, since historical behaviour of mmap(2) was to silently
ignore length-zero mmap requests; but mmap now returns EINVAL, which caused
look(1) to emit an error message and fail.

Among other things, this makes freebsd-update fetch on a newly installed
8.0-BETA3 system print bogus warning messages.

MFC after: 3 days