core(ocl): Don't query image formats when none exist

clGetSupportedImageFormats returns CL_INVALID_VALUE if called with
num_entries 0 and a non-NULL image_formats pointer so let's not do that.

modules/core/src/ocl.cpp

@@ -6445,6 +6445,8 @@ struct Image2D::Impl
                                                 CL_MEM_OBJECT_IMAGE2D, numFormats,
                                                 NULL, &numFormats);
         CV_OCL_DBG_CHECK_RESULT(err, "clGetSupportedImageFormats(CL_MEM_OBJECT_IMAGE2D, NULL)");
+        if (numFormats > 0)
+        {
             AutoBuffer<cl_image_format> formats(numFormats);
             err = clGetSupportedImageFormats(context, CL_MEM_READ_WRITE,
                                              CL_MEM_OBJECT_IMAGE2D, numFormats,
@@ -6457,6 +6459,7 @@ struct Image2D::Impl
                     return true;
                 }
             }
+        }
         return false;
     }