Fix off-by-one error (discovered by Shao Miller).

src/net/tcp/iscsi.c

 	/* Process challenge an octet at a time */
 	for ( ; ( value[0] && value[1] ) ; value += 2 ) {
 		memcpy ( buf, value, 2 );
-		buf[3] = 0;
+		buf[2] = 0;
 		byte = strtoul ( buf, &endp, 16 );
 		if ( *endp != '\0' ) {
 			DBGC ( iscsi, "iSCSI %p saw invalid CHAP challenge "