graphics/opencv: unbreak with clang 7

modules/stereo/src/descriptor.cpp:229:34: error: ordered comparison between pointer and zero ('const int *' and 'int')

	     CV_Assert(image.size > 0);
		       ~~~~~~~~~~ ^ ~

modules/stereo/src/descriptor.cpp:230:33: error: ordered comparison between pointer and zero ('const int *' and 'int')

	     CV_Assert(cost.size > 0);
		       ~~~~~~~~~ ^ ~

modules/core/include/opencv2/core/base.hpp:478:84: note: expanded from macro 'CV_Assert'
#define CV_Assert(...) do { CVAUX_CONCAT(CV_Assert_, CV_VA_NUM_ARGS(VA_ARGS)) (VA_ARGS); } while(0)

										    ^~~~~~~~~~~

modules/core/include/opencv2/core/base.hpp:455:35: note: expanded from macro 'CV_Assert_1'
#define CV_Assert_1( expr ) if(!!(expr)) ; else cv::error( cv::Error::StsAssert, #expr, CV_Func, FILE, LINE )

				   ^~~~

PR: 230355