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]

unix/gtkdlg.c

@@ -1226,6 +1226,7 @@ static void fontsel_ok(GtkButton *button, gpointer data)
 	(GTK_OBJECT(button), "user-data");
     struct uctrl *uc = (struct uctrl *)fontsel->user_data;
     char *name = unifontsel_get_name(fontsel);
+    assert(name);              /* should always be ok after OK pressed */
     gtk_entry_set_text(GTK_ENTRY(uc->entry), name);
     sfree(name);
 }