When you declare a foreign function to accept an
argument of type :cstring, this is converted into a
ecl_string_pointer_safe() call. However, this function
does not make sure the given string object is NULL
terminated, which can happen with non simple strings
(typical example: the result of a (format nil...) form).
This is a problem for functions expecting a NULL
terminating string (and it's not UFFI compliant).