inet: detect short TCP option lengths

Previously, a TCP option length of zero would cause inet to end up
in an infinite loop.

This resolves #7, reported by Alejandro Hernandez.

Change-Id: I45ad4c789d10d8e202cf6e140a7b9db7a6543c75

minix/net/inet/generic/tcp_lib.c

@@ -90,6 +90,8 @@ size_t *mssp;
 		if (i+2 > tcp_hdr_len)
 			break;	/* No length field */
 		len= cp[1];
+		if (len < 2)
+			break;	/* Length too short */
 		if (i+len > tcp_hdr_len)
 			break;	/* Truncated option */
 		i += len;