message: core(ocl): fix ocl::Image2d::isFormatSupported()
  in case of OPENCV_OPENCL_DEVICE=disabled
diff: @@ -6458,6 +6458,9 @@ struct Image2D::Impl
  CV_Error(Error::OpenCLApiCallError, "OpenCL runtime not found!");
  cl_context context = (cl_context)Context::getDefault().ptr();
  + if (!context)
  + return false;
  +
  // Figure out how many formats are supported by this context.
  cl_uint numFormats = 0;
  cl_int err = clGetSupportedImageFormats(context, CL_MEM_READ_WRITE,
  