Bugs in graphics functions

使用图像示例和分形示例重现故障，然后检查 arrayfire/graphics.py 中 Window.image 调用附近的代码，以及 arrayfire/util.py 中 to_str 和 safe_call 附近的代码。跟踪图形操作返回的后端错误；当两个示例都能在没有报告的 AttributeError 的情况下运行并显示其输出时，即表示完成。

This my code:
`
X = af.image.load_image(F[0],is_color=True)

win = af.Window(1280, 768, "imshow")
win.image(X)
`

It is throwing following error:

AttributeError Traceback (most recent call last)
in ()
----> 1 win.image(X)

/home/narsi/anaconda3/lib/python3.5/site-packages/arrayfire-3.5.0-py3.5.egg/arrayfire/graphics.py in image(self, img, title)
139 """
140 _cell = _Cell(self._r, self._c, title, self._cmap)
--> 141 safe_call(backend.get().af_draw_image(self._wnd, img.arr, c_pointer(_cell)))
142
143 def scatter(self, X, Y, Z=None, points=None, marker=MARKER.POINT, title=None):

/home/narsi/anaconda3/lib/python3.5/site-packages/arrayfire-3.5.0-py3.5.egg/arrayfire/util.py in safe_call(af_error)
77 err_len = c_dim_t(0)
78 backend.get().af_get_last_error(c_pointer(err_str), c_pointer(err_len))
---> 79 raise RuntimeError(to_str(err_str))
80
81 def get_version():

/home/narsi/anaconda3/lib/python3.5/site-packages/arrayfire-3.5.0-py3.5.egg/arrayfire/util.py in to_str(c_str)
70
71 def to_str(c_str):
---> 72 return str(c_str.value.decode('utf-8'))
73
74 def safe_call(af_error):

AttributeError: 'NoneType' object has no attribute 'decode'

Getting same error with fractal example too.