read_ns_sys_file(): don't overrun buf

This is a classical off-by-one error. If sizeof(buf) is 512,
the last element is buf[511] but not buf[512].

Note that if read() returns 0, we return 0 but buf stays
uninitialized.

Reported by Coverity, CID 114623.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

net.c

@@ -45,8 +45,8 @@ int read_ns_sys_file(char *path, char *buf, int len)
 	rlen = read(fd, buf, len);
 	close(fd);
 
-	if (rlen >= 0)
-		buf[rlen] = '\0';
+	if (rlen > 0)
+		buf[rlen - 1] = '\0';
 
 	return rlen;
 }