Fix another GtkFixed regression, in gtk_fixed_forall()

Fix another GtkFixed regression, in gtk_fixed_forall()

b3f6f67c changed the loop from while() to for() in gtk_fixed_forall(),
but that's wrong since the callback can have side-effects on the list,
in case the current child gets removed. And that's the case when the
widget is destroyed.

Patch by Vincent Untz
https://bugzilla.gnome.org/show_bug.cgi?id=641196