Add an assertion.

[bangerth]

The lines following where I add the assertion are only valid for the case where a cell is not further subdivided (i.e., build_patches() has not been instructed to subdivide the cell).

Part of #14403.

/rebuild

[masterleinad]

The following tests FAILED:
	 16 - simplex/data_out_write_vtk_01.debug (Failed)
	 17 - simplex/data_out_write_vtk_02.debug (Failed)
	 20 - simplex/data_out_write_vtu_01.debug (Failed)
	 44 - simplex/mapping_fe_01.debug (Failed)
	 45 - simplex/mapping_fe_fields_01.debug (Failed)
	 46 - simplex/mapping_fe_fields_02.debug (Failed)
	 50 - simplex/matrix_free_03.debug (Failed)
Errors while running CTest
	 88 - simplex/step-20.debug (Failed)
	 90 - simplex/step-31.debug (Failed)
	 91 - simplex/step-38.debug (Failed)
Error: Process completed with exit code 8.

[bangerth]

Hm, I clearly have to think about this some more then...

[bangerth]

This issue is more complicated. I'm working on a different patch.