failed assert when trying to overwrite save

Seems something broke the confirmation window to overwrite a file ("... does already exist... overwrite?")

Way to reproduce:
* Start a game (single/multiplayer does not matter)
* Open the save dialog with STRG+S
* select an existing save game from list
* hit enter (or click okay)

now widelands will hit a failed assertion:

widelands: /home/mronki/widelands/trunk/src/ui_basic/window.cc:146: virtual void UI::Window::move_inside_parent(): Zusicherung »parent->get_inner_w() > static_cast<uint32_t>(get_w())« nicht erfüllt.

bt from gdb is:

#0 0xffffe424 in __kernel_vsyscall ()
#1 0xb7aa6a81 in raise () from /lib/i686/libc.so.6
#2 0xb7aa83d2 in abort () from /lib/i686/libc.so.6
#3 0xb7a9fb08 in __assert_fail () from /lib/i686/libc.so.6
#4 0x083d3a7e in UI::Window::move_inside_parent (this=0xaed1f00)
    at /home/mronki/widelands/trunk/src/ui_basic/window.cc:146
#5 0x083c3013 in UI::Panel::set_size (this=0xaed1f00, nw=441, nh=140)
    at /home/mronki/widelands/trunk/src/ui_basic/panel.cc:266
#6 0x083c317e in UI::Panel::set_inner_size (this=0xaed1f00, nw=401, nh=100)
    at /home/mronki/widelands/trunk/src/ui_basic/panel.cc:323
#7 0x083b764c in UI::WLMessageBox::WLMessageBox (this=0xaed1f00, parent=0xae0f4d8, caption=..., text=...,
    type=UI::WLMessageBox::YESNO) at /home/mronki/widelands/trunk/src/ui_basic/messagebox.cc:78
#8 0x084d7449 in SaveWarnMessageBox::SaveWarnMessageBox (this=0xaed1f00, parent=..., filename=...)
    at /home/mronki/widelands/trunk/src/wui/game_main_menu_save_game.cc:208
#9 0x084d6c3e in Game_Main_Menu_Save_Game::Ok::clicked (this=0xae0f9dc)
    at /home/mronki/widelands/trunk/src/wui/game_main_menu_save_game.cc:247
#10 0x083bd041 in UI::Button::handle_mouserelease (this=0xae0f9dc, btn=1 '\001')
    at /home/mronki/widelands/trunk/src/ui_basic/button.cc:296
#11 0x083c3fac in UI::Panel::do_mouserelease (this=0xae0f9dc, btn=1 '\001', x=78, y=9)
    at /home/mronki/widelands/trunk/src/ui_basic/panel.cc:863
#12 0x083c4352 in UI::Panel::ui_mouserelease (button=1 '\001', x=78, y=9)
    at /home/mronki/widelands/trunk/src/ui_basic/panel.cc:958
#13 0x0825e182 in WLApplication::handle_input (this=0x86e1600, cb=0x86df4a0)
    at /home/mronki/widelands/trunk/src/wlapplication.cc:620
#14 0x083c2c12 in UI::Panel::run (this=0x88f4338) at /home/mronki/widelands/trunk/src/ui_basic/panel.cc:162
#15 0x082e0c49 in Widelands::Game::run (this=0xbfffcdc4, loader_ui=...,
    start_game_type=Widelands::Game::NewNonScenario) at /home/mronki/widelands/trunk/src/logic/game.cc:526
#16 0x08263f59 in WLApplication::new_game (this=0x86e1600) at /home/mronki/widelands/trunk/src/wlapplication.cc:2015
#17 0x08262f99 in WLApplication::mainmenu_singleplayer (this=0x86e1600)
    at /home/mronki/widelands/trunk/src/wlapplication.cc:1599
#18 0x082629ae in WLApplication::mainmenu (this=0x86e1600) at /home/mronki/widelands/trunk/src/wlapplication.cc:1523
#19 0x0825daa7 in WLApplication::run (this=0x86e1600) at /home/mronki/widelands/trunk/src/wlapplication.cc:401
#20 0x0825b81f in main (argc=1, argv=0xbfffeda4) at /home/mronki/widelands/trunk/src/main.cc:48