Unix: stop accessing ctrl->fileselect for font selectors.

The setup code for CTRL_FILESELECT and CTRL_FONTSELECT is shared,
which means it's a mistake to test ctrl->fileselect.just_button in it
without first checking which control type we're actually dealing with.

UBsan picks this up by complaining that the just_button field contains
some byte value that's illegal for a boolean. I think it's also the
cause of an intermittent assertion failure reported recently, in which
dlg_fontsel_set finds that uc->entry is NULL when it never ought to
be. If the byte from the wrong union branch happened to be 0 by sheer
bad luck, that could give rise to exactly that failure.

unix/dialog.c

@@ -2143,7 +2143,10 @@ GtkWidget *layout_ctrls(
           case CTRL_FONTSELECT: {
             GtkWidget *ww;
 
-            if (!ctrl->fileselect.just_button) {
+            bool just_button = (ctrl->type == CTRL_FILESELECT &&
+                                ctrl->fileselect.just_button);
+
+            if (!just_button) {
                 const char *browsebtn =
                     (ctrl->type == CTRL_FILESELECT ?
                      "Browse..." : "Change...");