Fix crash in GoPro.overview() when there is no SD
Fix crash in GoPro.overview() when there is no SD

When there is no SD the camera returns "0" as space left and then
`overview` method try to calculate its logarithm.

Error reported:
	Traceback (most recent call last):
	  ...
	  File "/home/fark/workspace/catec/gopro-py-api/goprocam/GoProCamera.py", line 1080, in overview
	    print("space left in sd card: " + "" + self.parse_value("rem_space",self.getStatus(constants.Status.Status, constants.Status.STATUS.RemainingSpace)))
	  File "/home/fark/workspace/catec/gopro-py-api/goprocam/GoProCamera.py", line 870, in parse_value
	    i = int(math.floor(math.log(size_bytes, 1024)))
	ValueError: math domain error

KonradIT/gopro-py-api,KonradIT/gopro-py-api