Fixed the font-selector demo so that it doesn't crash. The problem

Fixed the font-selector demo so that it doesn't crash.  The problem
was that when the family changes, it tries to initialize the face to
one with the same name as the selected one in the old family.  When
there is no such face in the new family, it got initialized to nil.

However, the demo is still not working properly.  Sometimes a
displayed face becomes non-clickable, and sometimes two faces are
highlighted simultaneously.