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]
ae802c16d8