Deal with the possibility of no valid font being selected at all

Deal with the possibility of no valid font being selected at all
during an entire run of unifontsel (because unifontsel_set_name was
either not called at all, or called with a name that didn't
correspond to any known font). In this situation we grey out the OK
button until a valid font is selected, and we have
unifontsel_get_name return NULL rather than failing an assertion if
it should be called in that state. The current client code in
gtkdlg.c should never encounter a NULL return, since it only calls
it after the OK button is clicked, but I've stuck an assertion in
there too on general principles.

[originally from svn r7953]