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

1 files changed, 4 insertions, 0 deletions

diff --git a/usr.bin/look/look.c b/usr.bin/look/look.c
index e6fd1b8..7c590c7 100644
--- a/usr.bin/look/look.c
+++ b/usr.bin/look/look.c
@@ -140,6 +140,10 @@ main(int argc, char *argv[])
 			err(2, "%s", file);
 		if (sb.st_size > SIZE_T_MAX)
 			errx(2, "%s: %s", file, strerror(EFBIG));
+		if (sb.st_size == 0) {
+			close(fd);
+			continue;
+		}
 		if ((front = mmap(NULL, (size_t)sb.st_size, PROT_READ, MAP_SHARED, fd, (off_t)0)) == MAP_FAILED)
 			err(2, "%s", file);
 		back = front + sb.st_size;