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.
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

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase