readme must not be 0-length

Reference: /n/sources/patch/applied/tareof
Date: Wed Sep  8 01:14:12 CES 2004

--- /sys/src/cmd/tar.c	Wed Sep  8 01:14:12 2004
+++ /sys/src/cmd/tar.c	Wed Sep  8 01:14:12 2004
@@ -724,6 +724,8 @@
 
 	if (recno >= nblock || first == 0) {
 		if ((i = readn(mt, tbuf, TBLOCK*nblock)) <= 0) {
+			if (i == 0)
+				werrstr("unexpected end of file");
 			fprint(2, "tar: archive read error: %r\n");
 			exits("archive read");
 		}