Explicitly check the result of a dynamic_cast.

[drwells]

Coverity also caught this.

[masterleinad]

Looks good (apart from the indentation 😉)!

[drwells]

Whoops. I fixed it.

[masterleinad]

/run-tests

[bangerth]

This patch yields the wrong exception. The documentation of the function states:

  /**
   * Return the matrix interpolating from a face of one element to the
   * subface of the neighboring element.  The size of the matrix is then
   * <tt>source.#dofs_per_face</tt> times <tt>this->#dofs_per_face</tt>.
   *
   * Derived elements will have to implement this function. They may only
   * provide interpolation matrices for certain source finite elements, for
   * example those from the same family. If they don't implement interpolation
   * from a given element, then they must throw an exception of type
   * ExcInterpolationNotImplemented.
   */
  virtual void
  get_subface_interpolation_matrix (const FiniteElement<dim,spacedim> &source,
                                    const unsigned int        subface,
                                    FullMatrix<double>       &matrix) const;

This means that the exception thrown after this patch does not match the description.

Can you fix this?