GtkTreeViewColumn: initialize a variable

GtkTreeViewColumn: initialize a variable

Shut up valgrind complaining about uninitialized values

==5330== Conditional jump or move depends on uninitialised value(s)
==5330==    at 0x624FF4C: gdk_window_move_resize_internal
(gdkwindow.c:6037)
==5330==    by 0x62502C3: gdk_window_move_resize (gdkwindow.c:6274)
==5330==    by 0x5E4FBB5: _gtk_tree_view_column_allocate
(gtktreeviewcolumn.c:2169)
==5330==    by 0x5E3846B: gtk_tree_view_size_allocate_columns
(gtktreeview.c:2631)
==5330==    by 0x5E43D76: gtk_tree_view_size_allocate
(gtktreeview.c:2686)
==5330==    by 0x7F03959: g_closure_invoke (gclosure.c:774)
==5330==    by 0x7F16446: signal_emit_unlocked_R (gsignal.c:3202)
==5330==    by 0x7F20256: g_signal_emit_valist (gsignal.c:3003)
==5330==    by 0x7F20421: g_signal_emit (gsignal.c:3060)
==5330==    by 0x5E6DFBE: gtk_widget_size_allocate (gtkwidget.c:4886)
==5330==    by 0x5D94ADD: gtk_scrolled_window_allocate_child
(gtkscrolledwindow.c:1506)
==5330==    by 0x5D95299: gtk_scrolled_window_size_allocate
(gtkscrolledwindow.c:1704)
==5330==    by 0x7F03959: g_closure_invoke (gclosure.c:774)
==5330==    by 0x7F16446: signal_emit_unlocked_R (gsignal.c:3202)
==5330==    by 0x7F20256: g_signal_emit_valist (gsignal.c:3003)
==5330==    by 0x7F20421: g_signal_emit (gsignal.c:3060)
==5330==    by 0x5E6DFBE: gtk_widget_size_allocate (gtkwidget.c:4886)
==5330==    by 0x5C62708: gtk_box_size_allocate (gtkbox.c:658)
==5330==    by 0x7F03959: g_closure_invoke (gclosure.c:774)
==5330==    by 0x7F16446: signal_emit_unlocked_R (gsignal.c:3202)
==5330==  Uninitialised value was created by a stack allocation
==5330==    at 0x5E4FA8D: _gtk_tree_view_column_allocate
(gtktreeviewcolumn.c:2139)