paned: minus the size by the handle for the for_child1 computation.

paned: minus the size by the handle for the for_child1 computation.

Fixes:
(rhythmbox:22802): Gtk-CRITICAL **:
_gtk_widget_get_preferred_size_for_size: assertion 'size >= -1' failed

that is gtk_paned_get_preferred_size_for_opposite_orientation calls
_gtk_widget_get_preferred_size_for_size on child2 with a negative
size.

As gtkpaned size was (32), child1 minimum size was (55) then
for_child1 became (32) for an handle_size of (5). Thus for_child2
ended up as (-5).

https://bugzilla.gnome.org/show_bug.cgi?id=705624