fix implicit sign conversion warning

[maxbachmann]

This is caused e.g. by:

__pyx_t_8 = __Pyx_PyObject_FastCall(__pyx_t_7, __pyx_callargs+1-__pyx_t_9, 1+__pyx_t_9);

from

        code.putln("%s = __Pyx_PyObject_FastCall(%s, __pyx_callargs+1-%s, %d+%s);" % (
            self.result(),
            function,
            arg_offset_cname,
            len(args),
            arg_offset_cname))

As an alternative the explicit conversion could be placed in each __Pyx_PyObject_FastCall call.