fix bug in gpu bgsegm sample

create `d_fgimg` matrix before cleaning.

samples/gpu/bgfg_segm.cpp

@@ -146,6 +146,7 @@ int main(int argc, const char** argv)
         double fps = cv::getTickFrequency() / (cv::getTickCount() - start);
         std::cout << "FPS : " << fps << std::endl;
 
+        d_fgimg.create(d_frame.size(), d_frame.type());
         d_fgimg.setTo(Scalar::all(0));
         d_frame.copyTo(d_fgimg, d_fgmask);