crash with weight system

[Miika Turkia]

Steps to reproduce:
- go to edit dive that has weight system set (double click on the divelist)
- double click on the weight system (e.g. belt)
-> crash

Program terminated with signal 11, Segmentation fault.
#0  0x000000000041439e in ws_edit_cb (button=0x0, w_idx=19136816) at
equipment.c:1154
1154		GtkListStore *model = weightsystem_list[w_idx].model;

miika